I'll add a bit to the linked posts, but I don't feel the need to repeat myself. Please do read them, as what I'm saying won't make sense otherwise.

First of all, if you are a peasant farmer who's been called to war, you almost certainly live in England, Scotland, Ireland, or one of the Scandinavian countries. The general levy ceased to exist on the continent in the 11th century, which is generally dated as the beginning of the High Middle Ages.

Second, if you are of a social class to be subject to the levy, you are among the better-off of the population. Freemen (petty land owners, tenants, and city dwellers) were subject to the levy; serfs, itinerants, and beggars were not. If you belong to this class, you would have received at least a minimal martial education, probably from your father, relatives, or neighbors: at least enough to know how to hold a spear and shield or draw a bow. You might very well have spent some time on active service, as a mercenary, a paid volunteer, a member of a lord's military household, or as a levied man in a previous war. In England at least, there was no mass conscription of all able-bodied men as in WWI or WWII; the king sent instructions to the county sheriff to raise X number of troops for the campaign, and the sheriff or his representatives went around the county recruiting until the quota was filled. As David Bachrach has shown, the same names show up again and again in different campaigns, so there was an element of volunteerism involved.

Third, the fact that you possess arms and can be called upon to use them on behalf of the kingdom is a valued right as well as an onerous obligation. Possession of arms has long been associated with status as a freeman. It's a chance to win glory and reputation, to prove your manhood, and demonstrate your status as a freeman. Beyond that, it's an opportunity to pick up a nice bit of loot and return home with the spoils. Beyond pillaging the countryside and sacking towns, the small beer of medieval warfare, a decisive victory would leave a field of valuables to plunder and captives to sell to your lords, who would then hold them for ransom. The odds are that you would make no great fortune, but more than a few ordinary soldiers returned rich from the wars.

We first have to address a problem here: What do you mean by 'peasant' in this usage? There's quite a lot that goes into raising an army as far as Medieval Europe is concerned, see.

Dealing with this specifically and the matter of peasants fighting, I've got three previous answers for your perusal. u/Rittermeister in this post explains why "What do you mean by peasant?" is relevant; u/MI13 examines who generally makes up Medieval armies; and u/Goiyon has a very comprehensive overview on Dutch armies of the period.