Amtgard is free to play but there are certain documentary requirements. These documents can be found under the Amtgard rules of play

If you want to play a monster class for a quest or a battlegame, with the approval of the game reeve, the monsters designed to go with are part of Dor Un Avathar

If you want a quick and dirty run-down on the rules for your class, there are always Mesreth's V8 cheat sheets. 

V8 Rules of Play and Mesreth's Cheat Sheets