More Health:

After pressing ~ to get the console type in xenux to get more health. This gives you a syringe for each corpse

Unlimited Ammo:

To get lots of ammo you need to kill an enemy with a sniper gun.(mission 6)(don't blow them to bits with a rocket launcher or you won't get infinite ammo.) then once he is dead go up to him were the body is. When a white line appears and corpse pops up on the top left corner keep pressing enter. Once the information sheet comes up just press [i] once your ammo is over 200 you should stop cause you will lose all your ammo