I am a citizen of Rome. Julius Caesar has just been assassinated; how long did it take for me to find out about this? What was my reaction?

This is a very broad question, which I'll try to answer as best as I can.

Rome was a pretty big place by the time of Caesar's death, so citizens throughout various parts of the Republic would have found out at varying times. Those in the Italian countryside would obviously have learned far more rapidly than those in Spain or Egypt. For the purposes of simplicity, I'm going to answer your question assuming you meant the city of Rome.

In the city, the response was almost immediate. Plutarch tells us that

Brutus stepped forward with the intention of making a speech to explain what had been done. The senators, however, would not wait to hear him. They rushed out through the doors of the building and to their homes, thus producing a state of confusion, terror and bewilderment amongst the people. Some bolted their doors; others left their counters and shops and could be observed either running to see the place where Caesar had been killed or, once they had seen it, running back again.

He then goes on to say

Brutus and his party, on the other hand, just as they were, still hot and eager from the murder, marched all together in one body from the senate house to the Capitol, holding up their naked daggers in front of them and, far from giving the impression that they wanted to escape, looking glad and condent. They summoned the people in the name of liberty, and they invited the more distinguished persons whom they met to join in with them.

Caesar was killed at the Curia of Pompey (not at the usual Senate House, which was undergoing renovations). You can see it on a map here (made in 40 BC). The Curia of Pompey was close to the nerve center of Rome, located relatively close to the Forum, Palatine Hill, and a number of heavily trafficked streets.

So the first non-witnesses would have learned about it within minutes of Caesar's death as a group of senators sprinted out of the Curia yelling about murder. Those senators ran to their homes, many of which would have been on Palatine hill, requiring them to pass through the busiest parts of the city, alerting citizens who then either rushed to their homes, to see the body, or to alert others, creating a cascade of people spreading the news. Shortly after that, Caesar's killers marched through the same section of the city, holding daggers covered in his blood and making people aware of what had happened.

Caesar himself had banned the use of vehicles on Rome's streets during the day, so the news would have traveled by foot (although in the chaos following the assassination, normal laws may have been violated). It's hard to do the exact math on how long it would take, but given the severity of the news, and the numbers of people who witnessed it, heard about it from a witness, or went to go so the body, most of the city would have been made aware within 2-3 hours.

As to how you would feel, again, that's a very complicated question. The simple answer would be, it would depend on who you were: you social and economic status, who you worked for, who you were bound to by marriage or social obligation.

As you can imagine from the fact that he was stabbed to death, certain people within the city did not like him very much. His killers generally fell into the camp of the "Optimates", a political party comprised of Rome's elite, advocating for the power of the patrician class. They had been threatened by Caesar's growing power, and many of them had fought against him. On the other side, Caesar's supporters came from his own party, the Populares. As you can infer from the name, they were a populist group who drew most of their supporters from the Plebeian class. Caesar was also extremely popular within the army, since he had lead them to a number of victories, which greatly enhanced the wealth and prestige of many of his men.

After Caesar's death, there was an uneasy truce. The Senate gave Caesar's body full burial honors, as well as announcing he'd be worshipped as a god. They also declared a general amnesty, and gave several honors to Brutus and other assassins. Brutus in particular, while not the ringleader (despite what Shakespeare would tell you) was a popular figure, and Plutarch tells us that he was respected by the people. These decisions were agreed upon by Marc Antony with one condition -- that he be allowed to eulogize Caesar.

At the time, accounts indicate that most people just wanted to avoid more bloodshed. Remember, this was just a few months after Rome had gone through four years of bloody civil war. People had their own private thoughts, but it seemed like a truce might hold. Three things would end up changing this: Caesar's will, Caesar's funeral, and Marc Antony's speech.

Suetonius tells us that in Caesar's will

To the people he left his gardens near the Tiber for their common use and three hundred sesterces to each man.

So that certainly helped the opinion of him. (300 sesterces would have been roughly 100 days wages for the average laborer).

When speaking about the funeral, Suetonius tells us that

Since it was clear that the day would not be long enough for those who offered gifts, they were directed to bring them to the Campus by whatsoever streets of the city they wished, regardless of any order of precedence.

The fact that they couldn't fit all of Caesar's funeral gifts into a single day suggests a large number of people, well beyond just his close friends and fellow elites. The "regardless of any order of precedence" also suggests that many of those bringing gifts were of the lower class. Many of those attending the funeral were those who had killed him, or who supported the killing.

During the funeral, and while the body was carried through the streets, it is noted that when the crowd saw Caesar's body (which had been very purposefully left so that the bloody wounds were obvious), people were greatly impacted, and decorum began to break down. This was furthered by Marc Antony, who made repeated reference to the body's wounds, and held up Caesar's bloody clothing at the crux of his speech.

When Marc Antony took the stage in order to eulogize Caesar, Arrian of Alexandria tells us that Antony noticed

wailing and lamentation arose again for a long time, and the armed men clashed their weapons, and very soon people began to change their minds about the amnesty. Then Marc Antony, seeing their state of mind, did not give up hope.

Antony went off script, praising Caesar generously. He then said

O Jupiter, god of our ancestors, and ye other gods, for my own part I am prepared to defend Caesar according to my oath and the terms of the curse I called down on myself, but since it is the view of my equals that what we have decided will be for the best, I pray that it is for the best.

It's a skilled piece of political rhetoric. He was very pointedly suggesting that he'd fight back against Caesar's killers if he could, without directly calling for their deaths and violating the peace.

When the Senate began to take offense at that, he said

It seems, fellow-citizens, that what has happened is the work not of any man, but of some spirit. We must attend to the present instead of the past, because our future, and indeed our present, is poised on a knife-edge above great dangers and we risk being dragged back into our previous state of civil war, with the complete extinction of our city's remaining noble families.

Antony ended his speech by waving Caesar's bloodied clothing in a passionate display. This was followed by several other dirge singers, but supposedly (According to both Arrian and Suetonius), what caused the crowd to break was a lyric from a song:

To think that I actually saved the lives of these men who were to kill me

At that point, the crowd broke. They seized Caesar's body, took it into the Senate House, and burned it down. Mobs rampaged through the streets, attacking the homes of Caesar's killers, and even brutally dismembering a man who just shared the name of a killer by mistake.

At that point, it seems reasonable to say that the popular opinion would have been very much on the side of Caesar (although it is good to note that people may have gone with the mob simply out of fear that they'd be next otherwise, or fear of the army).