Are there recruiting offices, or am I going to have to find an army camp? Is there some kind of selection process I'll have to go through?
A really huge and really interesting topic! The very long story short is that it all depends on what era of "Roman" you are. There are a number of distinct 'stages' of the Roman army, which evolved as much as the Roman state itself evolved. Let's start at the beginning, but I believe the question you were asking will be answered in the second bit of this post :)
The Kings of Rome: ~753 BCE - ~509 BCE
The founding of Rome is traditionally noted don as 753 BCE. We don't know the exact date - it could be a good bit on either side of that - however, that date is a good benchmark. Tradition (And Livy) states that there were seven kings of Rome. During those ~250 years of their reign, the kings of Rome were consistently at war (barring Numa) for the vast majority of their reigns. The Roman army during this period was incredibly similar to the two regions that the city was sandwiched between: The Etruscans to the north and the Greeks to the south (Magna Graecia). Their warfare was on a scale as small as Rome herself was; the Romans essentially raided their neighbours and were counter-raided in return.
All we have from this period, unfortunately, are the writings of men such as Livy, who wrote in the 1st century BCE. Much of it is untrustworthy, and it's rather difficult to sort out fact from fiction in this case, and we're honestly not even sure what date the Romans adopted the phalanx, or how often it was used. Remember - the phalanx was far more useful for pitched battles, rather than the raids that were pre-eminent in this period! :)
These hoplites were all landowners who were wealthy enough to purchase their own heavy armour and weapons, and were called up whenever the king called them to war.
The Servian Reforms: ~578-534 BCE (ish)
I'm including this one mostly because the results are significant throughout the history of the Republic, despite it being in the later part of the monarchy. The major result? The Comitia Centuriata, which would become the basis of Roman voting during the Republic. The way it worked was to divide landholders into tiers based on their wealth. Each of these tiers was subdivided into centuries - which, contrary to the name, were never composed of 100 men as far as we know. The tier list, and what they were expected to provide to the army, is as follows:
| Class | Property in asses (bronze coins) | Required Equipment | Number of Centuries |
|---|---|---|---|
| I | 100,000 | Helmet, round shield, greaves, cuirass, spear, sword | 80 |
| II | 75,000 | Helmet, oblong (oval) shield, greaves, spear, sword | 20 |
| III | 50,000 | Helmet, oblong shield, spear, sword | 20 |
| IV | 25,000 | Possibly an oblong shield, spear, javelin | 20 |
| V | 11,000 | Sling, stones, javelin | 30 |
There were 23 centuries beyond those; 18 centuries of equites, who made up the cavalry arm of the early Roman army, who were above Tier I in terms of wealth, two centuries each of engineers and musicians, and one century for the landless poor, who would not serve in the army.
If you wanted to join the army after these reforms (still a phalanx, remember!), then you just had to figure out what tier you were in. When the call to arms went out (literally every year), you would be obligated to serve for 16 campaigns before you were able to "retire."
This system would continue until the Samnite Wars, which were on-and-off conflicts between the Samnites (The mountain herders) and the Romans (the farmers) between 343-290 BCE. During those years, the Romans adopted the system that they would use for the next couple of centuries, and which would arguably produce the most effective militia in history.
The Manipular Legion: ~310ish BCE - ~late 2nd century BCE
Polybius goes into quite heavy detail on this legion, which is supremely helpful for figuring out exactly what was up. Just focusing on recruitment, there's a wealth of information to work from! Yay us! One thing to note here - this is the first period where the State occasionally armed troops during times of crises, such as after the disastrous Battle of Cannae.
So every year, the Senate would decide how many troops would be raised and where they would go. Depending on the military situation, that could change quite drastically; the best example of this fluctuation is the Punic Wars, where, at times there were hundreds of thousands of men fighting under the Roman banner. If you wanted to join the Roman army during this period, then congratulations! You're one among many patriotic young men who are lit by a fire of patriotism. Even though there was a good chance you would die in the Punic Wars, oh well. You were fighting for the glory of Rome!
Citizens in this time were still divided along those property lines that I mentioned before, but at this point, age was also a factor. The poorest (Tiers IV and V) were now known as the velites, and they fought alongside those who were too young to join the close-order infantry. Basically, if you were one of these guys, your job boiled down to chucking javelins at the bad guys. You would be equipped with a bundle of javelins, a sword (every man in the Roman army was a swordsman), a small shield, and a helmet or cap, which you were supposed to tie a piece of animal skin to.
If you were a bit older (~16-early 20s), you would be in the "front lines," known as the hastati. These were the younger men, eager to prove themselves for their families and for Rome. Behind the maniples of hastati would be the principes, who were from their mid 20's to their early 30's. Finally, in the back row, would be the triarii, who were the older veterans. The triarii would be equipped with a hoplite spear, and would provide the last line for the legions to regroup and fall back on, or for a desperate, final charge.
So in this case? Depends on how much money you have and how old you are. But you're going to be recruited - and often volunteer! - every year in the month of Mars. Today, we call that month "March." Men were levied from the available citizenry based on the census and the needs of the State.
Interim - The Punic Wars
Now, I mentioned the Punic Wars before, but they deserve another mention here. These wars were the first time the Roman State equipped men to send to war. Even if those men were otherwise too poor or ill-equipped to do so, Rome needed her fighters. In addition, Rome had some pretty impressive overseas conquests during this century, from Greece and North Africa to Spain and Transalpine Gaul. Conquests like that required a garrison, and the current system was stretched a bit to accommodate those garrisons. Men could expect to spend many years abroad when they were levied, and, as they were all citizen-farmers, rather than career soldiers, their livelihoods were generally in ruins by the time they got home. When they had their livelihoods ruined, they were forced to sell off their land. When they were forced to sell off their land, they were forced into that last tier on that list that I mentioned earlier - the landless poor. See how that one works?
Needless to say, by 146 BCE, the system was spiraling out of control. Property requirements for recruiting were being lowered across the board, soldiers were far less disciplined, and the more legions Rome was able to field, the more landless poor kept accumulating in the city of Rome itself. And that led to what we now call...
The Marian Reforms: Late 2nd century - the Reforms of Augustus
When Caius Marius was denied the ability to recruit men to head to war in Numidia, he went ahead and messed with the way things worked some more. Unfortunately, the way it happened was just a climax of the aforementioned spiral; Marius took anyone who would volunteer into 'his' army. This army was about as professional as the later Manipular Legion, however it was far more dangerous in a way. The volunteers for Marius' army were formed completely from that last century I mentioned a while back, which was known offhand as the capite censei. So when Marius came around and for the next century? All you had to do to join the army was to answer whichever general was recruiting. Head to one of his officers who was soliciting interest and profess interest.
In return, Marius (and the rest of the First Century generals) provided those poor with a steady income (even if it was low), they provided the weapons and armour they would use to fight, they provided the training, and they even promised to provide land upon the conclusion of the war - something these poor men craved. The trouble came when the Senate refused to give those general-politicians that land to parcel out. The soldiers, feeling betrayed by the State, were far more willing to follow after the general who had taken care of them thus far, and the general-politician in turn would lead them to his own interests. Those interests were often violent, chaotic civil wars, and Rome was rocked by civil war for the majority of the First Century BCE.
continued in the next post
Edit: Celebreth has a much better comment down the thread a ways. I highly recommend reading that in place of my post, but I won't delete mine because some people liked it.
Okay this is an awesome question! If you don't know anything about the Marian Reforms I highly recommend reading up on that topic to better understand the issues at hand. Basically, the way that the Roman military operated changed drastically after Marius tweaked recruitment and armament practices.
In the early days of the Republic, the military operated almost as more of a feudal militia. Landowning men (because who else was serious enough about strengthening and defending Roman territory if not landholders?) between the ages of 17 and 46 would be considered eligible for military service during times of war. Important to note is that before the Marian Reforms, Rome did not have a standing army. After the Reforms and the massive expansion of the empire, a standing army was required, particularly an army split up into small garrisons all throughout the empire that were capable of mobilizing into larger forces on short notice (read up on Comitatenses for more info on late Roman armies).
When Rome went to war, and Rome was nearly always at war throughout its existence with some group or another, a red flag would be hoisted over the capitol and the doors to the temple of Janus (not Juno!) would be opened. When the war ended, the doors closed. Anyways, when landowning Romans heard that they were mobilizing for war, 17-46 y/o landowners would report to Rome, where they would be reviewed by the tribunals/consuls. These state officials would select the men they deemed respected, wealthy, and fit enough for service. Care was taken not to recruit too heavily from a particular family, tribe, or geographical area so that a particular class or tribe wouldn't be wiped out in the event of a massive defeat. The men chosen for duty would be told to report to a staging area at a date in the near future. For example, during the Greek Wars recruits were instructed to gather at the far southeastern tip of the Italian Peninsula. These men would have spent the months before their departure purchasing armaments, horses, and slaves to carry their things for them. Keep in mind that the Marian Reforms are what standardized the army, and that much like medieval knights and man-at-arms the early Roman soldiers would have had to acquire their own equipment. These Roman soldiers would have fought as heavy spearmen in a Manipular formation, and had more in common with heavy hoplites than the later Roman army.
Now, post-Marian Reforms are a different story. I recommend reading the wiki article for Gaius Marius for a better understanding of the reforms he put in place. At this point, Rome is an empire, not a city-state, with its own distinct culture that has completely dominated the Italian Peninsula. Now that Rome has a standing army, recruitment is not limited to immediately before a war breaks out. Recruiting would obviously be much higher at these times, due to, essentially, patriotism. If you were interested in joining, you no longer had to own land. All you had to do was prove your Roman citizenship, typically by letter from a respected Roman citizen. You would go to what was basically a recruitment office, which would have existed in large Roman towns and cities through Italy. If the soldiers or statesmen deemed you worthy at the recruitment office, then you would be shipped off almost immediately to a legion garrison where you would be given your gear and training.
Any questions?
Sources:
Plutarch's Life of Marius
http://www.roman-empire.net/army/becoming.html
http://romans.etrusia.co.uk/roman_army_intro.php
(the sources aren't perfect, but it's been a couple of years since I've read much on this subject so I googled some stuff to refresh my memory! I can rummage through some books upstairs if further sourcing is required, though.)