Ammunition basics needed - links please?

I need some links for basic information about ammo. I have several different weapons and need to know what sort of ammo I would need for targets, and for home protection. I have used weapons in the military and done some crittering, but never really hunted. Would like to be able to hunt also. I just don't know what sort of ammo to start with; what is worth paying for, and what I need to keep around for bigger game. Thanks much