Honestly my young friend. You dont give us a lot of informations to help you.
A good website about infantry/army?
Is it from a technical point of view? historic? human? political? social? economic?
You have to give some informations to receive the right informations.
So, I say Wikipedia. You type army, infantry, and you will have tons of definition and informations... It may help you to get some answers.
You have imperatively to set your topic. Take this advice from me, I often guide students who work on their memoirs. Then start working from there. You can change your topic from there...
I advice you to search for the roots of the key word you care about. Infantry comes from "infant". What's the meaning of that? why? where is the link between infantry and infant at that time? and then, how it evolved to nowadays? ...
Always a good introduction.