On the main screen hit main menu, sign in 2 accounts (one can be a guest), and go into either custom games or matchmaking. Custom games is a usually local (played with only people on your xbox) and customizable gametype. Matchmaking is played with anybody on xbox live (so you must have xbox live gold to play) and gameplay is generated from the online game server and is voted on by members in the game type. Each of these will be split screen automatically if you have 2 people signed in.
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Depends on what you like, I favor Halo and Call of Duty.
no you cant
Yes...
You change the play list in Multiplayer.
Halo Reach is definitely for single player. Campaign and Forge, you can do alone, but you will get the most amount of entertainment out of the multiplayer modes.