how do i level

Hey I havn't played this mage since TBC, and I'm noticing the class has changed quite a bit. I preferably want to level as fire but what should my rotation be on basic mobs/instances? Basically tl;dr: how to level as a mage
Umm leveling as fire will have you spamming fireball for most of the time, and a couple of aoe spells.

But until you get talents like hot streak and abilities like living bomb, fireball will be the spell your casting most of the time. flame strike is okay to use again once the floor dot is gone for aoe, but you really don't wanna get into the habit of spamming it.
The second tier talent "impact" will have you spreading ignite and other dots to nearby targets when you use fireblast. (And for the love of god, new mages please stop hard casting pyroblast every cast)
Depending on how fast mobs die, you may also be able to just scorch spam. If I were you, I'd get improved scorch talent so it is free to cast.
Target an enemy, cast Pyroblast. Cast fireball. Cast the last fireball (assuming it isnt dead already). For elites its a little different and really you wont have access to a lot of the core abilities that make fire awesome till the 60s.

That said, as fire you will still be 2-3 shotting normals all the way to northrend. Then its 5 or 6 shots if you are unlucky.

Once you have the abilities youll want to start a fight off with either a scorch or a pyroblast to apply critical mass which increases the crit chance of all fire spells. Second you want to put on living bomb (gained at 69), and then flame orb (gained at 81). From there its fireball spam till you get hot streaks which you use immediately and impact procs which you use to spread dots to multiple mobs. The only tricky thing to use is combustion (speced into at minimum 39). The reason why combustion is tricky is because you want to use it when you simultaneously have critical mass, a large ignite (the most important thing), Living bomb (second most important thing), and pyroblast's dot (least important thing) on the target. There is an add on that almost all mages use called combustion helper to make sure combustion is used at the right time.