Well the first thing is friends, you should want a big friend base whether its online or local it doesn't matter considering you can play custom games with both. Obviously if you don't have people to play with your done.
Next is to make or use a map for custom games. Most custom games are played on personally made maps customized in Forge. You can also just take someone else's customized map if you like it through file share.
Finally go into the Custom Games section from the Halo 3 mainscreen, find and load a game, map and settings and either invite friends through Xbox Live to join your game or have some local friends plug in their controllers.
To also add to you map list you can dowload some free map packs for Halo from the Xbox Live Marketplace.
