As rmnelson suggested, go to this site and read up:
They have a section that will educate you on the four C’s: Cut, Color, Carat and Clarity.
And actually, that site offers decent pricing too, if you don’t mind making a purchase like that on the internet – I believe they have an excellent return policy if you’re not satisfied.
I just bought a ring myself about a year ago, so I know how you feel.
In my personal opinion, color and cut are the two most important things – they determine most of how the diamond looks up close. Clarity is overrated, unless you need to re-sell the stone (knock on wood of course) – you need to look under a 10X magnification lens to see flaws in a decent quality stone, and making it so you have to look in a 100X magnification device doesn’t really do much for the ring. Carat (as in size) is important too, but I have a personal aversion to big, flashy stones, so I maximized the quality I could get in the other three categories while sticking to my price range and my pre-set “non-flashy” limit for size (this will depend on the size of your girl’s finger too – luckily, my petite fiancee has a size 3 3/4 ring finger…).
As brider said, it’s much better to get a custom design – and also to buy a loose stone. Of course, it depends on your budget, but you should be able to find a jeweler who will work with you no matter what you want to spend.
Someone else also had some good advice I want to echo: Find out what she wants first, then get the ring. Yeah yeah yeah about “she should like what I get her” – all fine and good, and definitely in terms of the cost of the ring. But in terms of the style you should see if she likes round cut vs. princess, one-stone setting or multiple, etc. You’re going to spend a good deal of money on it (irrespective of the amount you spend, I’m sure it will be an amount you consider to be a lot), and it’s for her, so you might as well make sure you’re getting something she wants. If you don’t want to ask her directly, the time-honored tradition is to get her friend or sister to do the recon and report back.
Best of luck.
ADDENDUM: I should add that if you are going to get a nice stone, make certain to get a GIA (Gemological Institute of America) certificate for it.