At 16 you have begun to pick a spec. Frost, Fire, or Arcane.
You should be using your main nuke (Each spec gives your main nuke a boost in tier 1) You should be picking all from the same spec too - do not put 1 point in each. Spec Fire, Frost, or Arc. Not a little of each.
At max range (or close to max) open with main nuke. You should have enough time to fire off 1 or 2 more main nukes before it gets to you.
When it arrives in melee range - Frost Nova, then run backwards. At this point - either main nuke or wand it down.
As you get more instant casts and shorter timed casts (Scorch) you'll have more options.
Fire Rotation:
Open - Fireball
Fireball
Fireball (Probably getting hit as spell goes off)
Nova
Get some distance
Fireball, Fireblast, or Wand.
Frost Rotation:
Open - Frost Bolt
Frostbolt
Frostbolt (You might NOT get hit at this point - frost slows the mob)
Nova
Get distance
Frostbolt or Fireblast or wand...
Arcane Rotation - This one is more complicated (I'll explain below)
Open - Frostbolt
Arc. Missile
Arc. Missile (You will probably get hit at this point)
Frost Nova
Move away
Fireblast, wand - whatever just kill it.
Why is Arcane a wierd rotation? Fire spells do more damage with Tier 1 talents. So - Opening with fire is ok - you don't get the slowing bonus of frost, but that is offset by the greater DPS. Later in fire - you'll open with Pyro. But you are a few lvls from that... Frost spells do more damage with Tier 1 spec - Plus slow the mob. Thus they get double the bonus. (FYI - that's why frost is considered the Leveling and control spec)
The Tier 1 talent for Arcane is Zero interuption - You get no buff to Damage. Arcane's main nuke is very strong, but it's a channelled spell - thus you cast the channel and then it fires off several bolts over the life of the cast. Any hit will likely break the channel - therefore stopping you from firing more Missiles. The tier 1 talent will avoid this, but doesn't offer any extra damage. Opening with Frostbolt applies the slowness. The mob, will then turn and march towards you. This will allow you to have the max time to cast your channel. With tier 1 talents - you'll never break channel -- However from an efficient standpoint - you'll do best to apply the frostbolt - THEN fire 2 Arc. Miss