if you have a hard drive and are connected to xbox live,just insert the game and it'll downloadthe backwards combatible update and the xbox game will play. and not all xbox games can be played on the xbox 360.
As xbox09 said not all games will work.If your unsure which games are compatible this is the link for Microsoft's list of backwards compatible games http://support.xbox.com/en-US/games/play-original-games
