Theo's advice is sufficient for level 69. He's also a douche to lots of people; don't take it personal.
A less "douche bag" answer: Linky
The most basic priority for single targets will be something like this:
1. If your pet is on the target, Kill Command
2. If the mob is going to live long enough, apply Serpent Sting.
3. If mob is below 20% health, Kill Shot
4. If you have enough focus to cast Kill command on CD, Arcane Shot
5. Cobra Shot
In a given 6 second cycle you should be able to fit 1 Kill Command, 1 Arcane Shot and 2 Cobra Shots with any excess time from shortenec casttime going to extra arcane shots over time.
I have no idea if any of that is applicable to a 69 hunter, as this is my hunter
(hint: note the gear).
Also there's a couple of stickies...
Edit: For future reference this would be the kind of question people around here would encourage you to find your own answers to. Seriously, check out some of the links in the stickies as well. Edited, Feb 1st 2011 9:51am by someproteinguy
That monster in the mirror, he just might be you. -Grover