As already mentioned this question is so vague it is totally unanswerable but I'm not going to get another chance to write about what I'm about to and I feel I may be of some help.

It is understandable that in many ways the study of war can seem somewhat strange, a basic history of a war will likely only contain information about battles and generals and will not cover the 'nitty gritty' of how exactly they came to be where they were or how they went about deciding where to go.

The Thirty Years War was fought from 1618 to 1648. On one side were the forces of the Hapsburg Holy Roman Emperor and his allies on the other their enemies who over the course of the war included the Bohemians, the Palatinate, the Danes, the Saxons, the Hessians, the Transylvanians, the French and most famously the Swedish. For the purpose of simplifying this question I won't mention why they were fighting but instead the systems through which an army would be recruited, organised and led.

The basic organisational unit in an army was the regiment, these were raised in a uniform way by most of the belligerents, a Colonel would be provided with money and in exchange would promise to raise a number of troops. They would then subcontract this task to a number of Captains who would each raise a company, sometimes a Captain would be paid to raise a 'free company' that would then be used as a garrison unit or as a way for an individual to show their skill at recruitment and thus achieve promotion. A regiment would consist of between 5 and 20 companies with 10 being seen as the ideal, a throwback to Ancient Rome. A company consisted of about two to three hundred men with the Captain being assisted by one or two Lieutenants and an Ensign who was responsible for the flag.

On the battlefield itself the regiment would be deployed in a tight formation, keeping it in sight of its mounted colonel who could use the centrally placed drummer to issue commands. Both sides in the war recognised that around a third of all recruits had to have some experience in warfare to train the others on the march and keep the formation steady in battle but no colonel wanted to lose experienced troops from an active formation. It was also accepted that larger regiments were more useful since they could operate on their own on a battlefield without having to be brigaded with other units and were less likely to be disbanded to be used as replacements for the inevitable casualties that the army as a whole would suffer on campaign.

There was no standardised career path for one to become a general. Rulers of states usually possessed the wealth required to raise an army and could choose to command it in the field, as Gustavus Adolphus did, but frequently did not. Albrecht Wallenstein was the commander of the Imperial Army in 1625. He had used money gained through marriage to raise two regiments for the Emperor at the start of the war and was involved in the occupation of Bohemia after its defeat, he grew astronomically rich through this and was able to buy a Duchy off the Emperor for 1.6 million florins (this is not really translatable into 21st century coinage but 10 florins could buy a person enough grain to last them a year in 1618). In June 1623 Wallenstein offered to raise an entire Army for the Emperor and by 1626 he had raised 70,000 troops. They were split between garrison duties and preparing to repel an invasion by a Protestant named Mansfeld.

One of the greatest problems encountered by states in this war was pay, armies were expensive to recruit and maintain. A general could only be successful if they were able to both provide capital to colonels for raising a regiment and a guarantee that the men they raise would be paid throughout their service. The system by which this was done relied heavily upon imposing taxes on the local populace, a colonel was permitted by the Emperor to seize food and shelter from locals at whatever rate they felt was necessary but still pay arrears quickly became enormous, the Swedes demanded 20 million florins to pay off their soldiers at the end of the war and the war itself was extended as both sides sought to find ways to avoid paying these vast sums.

I could write more but I'll wait to see if there is any interest before I do. Obviously if there are any follow up questions I am more than willing to answer them.

Could you narrow this down to a particular time period, war, nation, etc? This is an extremely broad question.