you should get to know your job, be it defense or offense. Protect your healers, counter-spell and/or sheep enemy healers.

To some extent, you can make use of AoE spells on groups of enemies and to defend flags. Flamestrike, Arcane explosion and Blastwave will stop a flag capture, Blizzard will not!

Likewise you should focus fire on a single target, but be aware of your wow powerleveling surroundings. Watch for healers that are hidden to the sides, a well placed polymorph can turn the tide of battle.

In part, it is nessary for you to remain hidden or in the back of your group if possible. Mages are a high value target since you can bring very high damage to battle. Expect Hunters and Rogues to target you.

The 'Bolt' type spells offer good damage at long range, but point back to you as a high-damage but soft target. Consider using scorch when you want to keep a low profile. It is fast casting, has an increased crit rate(with talents) and can generate a stacking damage-increase debuff (with talents).

In most cases you will want to have and keep range against melee targets. Warriors will charge and rogues will sneak up and ambush you. Frostnova will lock them in place and allow you to move away.

Sheeping a melee target will allow you to take a moment to cast your larger damage spells.

When the spell is almost done casting, start hitting the button to cast a fireball. The second spell should be nearly done casting as the first one hits. And melee targets will move toward you, so it is best to keep moving and use Scorch and Fireblast to finish them off. So it is important to save your Blink to escape from stuns and intercept.