Going to see the eye doctor?!

They'll tell you then and there what's going on with your eyes - they do several different tests (one for vision, one for glaucoma, etc), and each test tells them something different. At the end of the appointment they'll give you a prescription for glasses, or contacts. Don't worry, the tests don't hurt. You will look into lights, read a chart, and probably get a quick puff of air in your eye (to test for glaucoma). They may also want to use drops to dialate your eyes, I'm not sure what this tests for though. Personally, that test makes me want to pass out - but i'm squeemish. If you're not comfortable with things in your eyes, I'd see if you can opt out of that one.

As for the prescription, if you get contacts they should be able to fit you at the appointment. Glasses will take a few days for the lenses to be made. I'd go with contacts if I were you - they're kind of nerveracking and frustrating at first, but sooooo worth it - much better than glasses. (Even if you don't like stuff in your eyes, these are easier to get used to than the drops, in my opinion).

years of bad eyes, eye doctor appointments, glasses, and recently contacts.

You will get a prescription for glasses on the same day (although it may take a few days for the lab to make your lenses, depending on where you go and what insurance you may be using). If you would like contacts, they will do some additional tests to determine that prescription (it will be different from your glasses prescription) and you may get a trial pair of lenses to wear out the same day.
The visit will also look at the medical health of your eye to see if there are any diseases that are affecting your vision. (@KatieM - the dilation opens your pupils wider so the doctor can see a larger part of the back of your eye to check for any irregularities or diseases). If there are any indications of problems with the eye, you will be referred to the appropriate specialist.