this instructable will show you how to make a simple, but far firing catapult in garry's mod 10. it is a common contraption that is perfect for beginners. most Garry's Mod players know how to make something like this.
Garry's Mod 10 - http://store.steampowered.com/app/4000/
a suitable source game (Half life 2, portal, counter strike, TF2 etc...)
Stacker STOOL - http://garrysmod.org/downloads/?a=view&id=23683
Smart weld - http://garrysmod.org/downloads/?a=view&id=18508