Don't really worry too much until you hit 50. But in general 1-2 mobs you will use a single target rotation (thunder fire 3{not sure when you get that} fire 1 until your almost out of mp then blizzard 3 or transpose of you don't have that yet). 3+ targets fire 3 to max stacks fire 2until your almost out of mp then blizzard 3 or transpose of you don't have that yet

On phone it is a bit more than that