Well that all depends on how you want to play.

The short version is: Frost= PvP and PvE Power Leveling.
                      Fire= PvE dungeons and steady Leveling               
                            (absolute wost PvP spec in the game).

                      Arcane= Great secondary spec when leveling,
                              once you start doing endgame stuff
                              and getting 1337 purple gear this
                              becomes THE damage spec.