You can't. In order to rank up, you need to raise your "highest skill level" and your "exp", which you can only do through matchmaking.
Matchmaking
You do not start off as a rank because there is no online matchmaking for ODST
halo 3 odst, unlike halo 3, has no matchmaking, sorry
General grade four
You play a lot of matchmaking games and over time you gain XP and rank up. This requires Xbox Live, if you didn't know. (If you didn't know that, that is very sad)
To play on Halo 3 matchmaking you must have a gold xbox live membership and an internet connection to play on matchmaking. If you only have a silver membership it will not let you play matchmaking.
No. You have to play on ranked, or social matchmaking.
No not unless you were banned from Xbox live all together.
play in matchmaking
Shoot and kill people.
keep doin matchmaking.
When you boot up Halo 3 go to the main menu, then choose matchmaking, but make sure you have Xbox Live connection, and there you have it, online multiplayer.