I know this, and any question regarding the medieval period is quite hard to answer, as "medieval" refers to several different cultures and governments at several different points in time. So, if you need a more specific place and time, consider 12th/13th century England and France (But, obviously not limited to these.)

In modern military forces, an individual is usually a volunteer or is obligated to military service through some sort of Forced Conscription system. However, large armies in medieval times, as I understand, were put together based on the feudal obligations of different lords and knights - Banner men, basically. But where would they actually gather these people? Were they gathered as volunteers from villages? Did you have to be wealthy in order to serve under a knight/Lord? Could the volunteer directly to the Lord? Would A village elder be ordered to muster a number of men from a village to join an army?. Would a lord give their units money to purchase weapons and armor, or would these men have to purchase them themselves?

I'm more asking how this would be done at the very first level of mustering a force. Like, if I'm a peasant, how do i go about becoming a fighting-man. I greatly appreciate any response, and ask for your patience in my ignorance in any of these subjects. Thank you very much.

Further reading sources are also greatly appreciated.