This happened a few days ago...
and as a result I've been trying to figure out how to gear her and get ready to raid. So I figured I'd write out a quick how to guide for any level 85.
First and foremost.... hit Ask Mr Robot. This lovely site works as a great starting point. No matter what class or spec, Mr Robot will give you some idea of where to start gearwise.
Step 1) If your character is not yet 85, I would suggest picking class and spec and seeing what's BIS. This should allow you to do some preplanning, like picking up crafted items/recipes on the cheap, and questing in certain areas for rep.
Step 2) hit 85. CONGRATS!
Step 3) Learn to play. Yup, this is the hard bit. So hard, in fact, that I broke it up into substeps.
a) If possible, see what your new class is required/expected to do in raids. For example, as a hunter I would be expected to slow adds on some fights, as well as use tranquilizing shot. Also, I'd be expected to mass-rez everyone after we wipe (yay guild perk!).
b) Hit elitist jerks, class forums, wow insider, and class blogs. Read and experiment. Pick up as many tips and tricks as you can. Look at what your class is doing on World of Logs too.
c) Practice! Hit the dungeons. Go PVP. Spend some time at the target dummies. Get used to playing your new class, and your new role. Now, some stuff (like raid healing) isn't something you can practice outside raids, but you can get used to your spells and get comfortable with them.
d) Remember, Rome was not built in a day. It'll take time to get used to your new role, so don't beat yourself up if you're not the BEST EVER in your first raid.