Hi mate! I've also got a Mage around those levels, and I tried PvP at 19 and didn't do very well at all.
The most people in 10-19 BGs are usually Warrior, Rogue or Hunter twinks. Those are the three classes who do best in the 19 bracket. The problem with mages at that level is that they haven't got their best abilities yet (as with any class, really).
When you hit 20, you get Blink, Evocation, Mana Shield and other such goodies:) My advice to you is to keep leveling and try the 29 bracket, you'll do much better there.