Will you play the campaign of your chosen faction? Or will you plunge straight into multiplayer? Will you do it alone or with friends?
Myself, I'm probably going to do the same thing I did when I got Halo: Reach, which was to complete the entire campaign solo first, which for me served to acclimate myself to the game's physics and such before I got my butt kicked in multiplayer. Once I finish the campaign, I'll throw myself into multiplayer and join up with any friends I see are playing (and nag my other friends into buying the game and joining me ).
How 'bout you?