at lvl 21 you really dont need to worry about it just yet.... the spells you know are really limited.
the beginning talents in all 3 talent trees basically work the same.
Generally, if you are new to WoW (or MMORGP) and chose a spell caster like mage to start with, it`s quite challenging.
For future reference, frost works the best for PvP, because of its relatively higher survivability and CC; Arcane has super high damage in short term but cost quite a lot of mana; fire is very good in PvE and does a fair amount of dps as well.
personally, I lvled my mage to lvl 52 with as fire, then switched to frost. I wanted to try out arcane long time ago but i was fully geared at lvl 70 with all the +frost dmg stuff. But now the expansion changed everything to just `spell power`so it doesnt matter anymore... i switched to arcane....