Curious as to gun laws? How to obtain Class Three License?

I recently turned 18 and am very interested in buying a gun, I'm American and I'd like to practice one of my fundamental rights. I'm also joining the Army and would like to have some experience with firearms before I join, i've shot guns before but I'd like to own one. Anyway, I want to buy a pistol (most likely a baretta) and I always wanted to buy some sort of M4, or one of it's variants. I've been doing my research and it appears that the closest thing to a M4 I can get is some sort of Bushmaster variant, I believe the AR15 or XM15. Anyway, I was wondering if these guns are considered to be assault weapons, and if so ive heard you have to obtain your class three license to buy these guns. So I'm curious as to what exactly I need to do to obtain this license, and just any other important information you guys can think of. Thanks a lot.