Start with a .22. They're cheap, ammo's cheap, and if you turn out to be a sound pussy, they're not that loud.
Then there's action style - bolt, pump, lever, semi-auto, and a few break-open. Bolt action's the classic sniper style. Lever is a cowboy gun (lots 'o fun IMO). Break-open and pump action make you feel like you're using a shotgun. Finally, there are semi-auto replications of most modern military guns. Plus if you fan the trigger, you can imagine you've got a full-auto
