Welcome to the forum, for your first post it is best to put this 'Mage Related' post into the Mage class forum:
Here To quickly answer your question, while leveling I would either pick fire or frost, later on for raiding arcane can make a fine supplement to either fire or frost for the increased damage and larger mana pool. Many people use less arcane as they get higher damage amounts in either +fire or +frost and go to more of a 10/48/3 (fire) or 10/0/51 (frost) builds for raiding. Once you get much farther into raiding arcane comes back into play when you get 2 pieces of Tier 5 (from SSC and TK raid dungeons).
Assuming that you are pretty far from this point I would recommend posting in the mage forum and reading the FAQ + Sticky there at the top of the page, very helpful information there.