The FIRST thing to do is decide on a budget. Determining how much you want to spend can have a big impact on things.
Next you want to find out what cut she wants. My wife wanted Princess cut, but most girls I know want a Round. The cut is actually pretty important as it is what determines the sparkle. Round and Princess give the best sparkle, as I recall, because they have the most facets for light to bounce off of. Round is almost always going to look bigger and sparkle more. Princess needs to be cut with a little more depth in order to get the same sort of shine, and from what I saw when I was buying you get more for your money with a Round, at least in the way it looks on her finger.
I would not get anything below a VS2 or a G-H color. You're going to start to see the flaws with the naked eye at that point.
As far as the setting goes, you need to decide if you want it to be a solitaire or if you want something with side stones. I strongly recommend that you do NOT get a band with pave set stones that go all the way around the ring, or even very far down the sides, as it will make it nearly impossible to have it sized. Personally I think solitaire is the way to go if you get her something above 1 carat, because it's large enough that a solitaire setting will give it prominence. If it's smaller than that it may be nice to flash up the ring a bit so that it's eye catching, but make sure the jewels on the band don't overpower the central diamond.
Anyway, I would avoid Tiffany, they're going to charge you like twice as much as you can get at any other jeweler.
I got my wife a 1.5k Princess, VS1, F diamond with a white gold band for around $9k in 2009. I ordered from Blue Nile and I think I got a good deal, but I sort of regret not shopping around the jewelry district here in the city a bit more.