Alright this happened to me before and its pretty simple
Wait for your xbox to load your information in the main menu of your Halo 3 game (not the dashboard)
Then press a on Matchmaking
Find the Play list option
if you want to play by your self choose Lone Wolves or Rumble Pit(Free for All)
but since your a beginner in Halo choose Basic Training and thats where you can figuire out what to do.
If you want to play with teams choose Team Slayer or Social Slayer (both are 4v4)
Alright i hope you have fun with it even tho i ddnt add all the playlist like MLG, Team Snipers, Team Objectives, Squad Battle, etc