How do you get ammo on gears of war

The simple answer to this is. Either pick up a weapon that's the same as your weapon to get it's ammunition. Or search for the blue C.O.G ammo crates. They are placed all over the campaign maps. Sometimes you will come across little magazines that don't give you as much ammunition as the ammo crates. If you wait long enough after picking up an ammo crate or Magazine it will re spawn within a short time.

Hope this helps