You don't ask, medic class gives health, assault class gives ammo. If you can’t fine either use the assault class yourself and you can give yourself ammo using LB. Oh, each class gives up points for upgrades. Just read your different classes its on the disc.
Yeah, it's the back button. Good luck getting anyone to give you ammo/HP though. I was standing in front of a guy knifing him and mashing the back button for 2 minutes trying to get ammo. If possible, it's better off to just play with people you
know and communicate.