Instructions
Go to the multiplayer area of the main menu. In Halo 3 and Halo 3:ODST, this menu can be reached by selecting "Multiplayer," then "Matchmaking." If you are playing Halo: Reach, you can select "Matchmaking" from the main screen.
Select "Find new match" or "Create new Match." Then select "Public" as the domain. In all Halo games, Public is listed as default, so you need not change this setting if you have not played in a private match recently.
Wait for the matchmaker to find players. Halo 3 and Halo 3: ODST may take a bit longer to create matches, as they are the oldest of the currently active Halo games. Once enough players have been found, a countdown begins. When the countdown finishes, the match starts.