I’m sure there are others who will reply with more info that I have, but you’ll want to start with the 4-C’s. Color, Cut, Clarity, Carets.
Don’t get overwhelmed by all the information the dealers will throw at you. Take your time and study them (both rings and dealers). Do not be rushed into anything, especially because they say it’s on sale. That’s the best way to pay too much.
Cut - The shape of the diamond. This is the one area you really need to find out her preference. Some women like them simple, some elegant, some flashy. I’ve always been fond of the Marquis cut myself as it looks larger than it really is (see Carets below) and makes a statement.
Color - Unless you want to get fancy and flashy and really expensive with colored diamonds (blue, rose, etc) it’s best to stick with colorless, or as colorless as you can afford.
Clarity - Again depends on the price you can afford, but as long as there are no flaws visible with the naked eye or simple jewelers loop, you’re doing OK. Flaws that you need special instruments to detect, aren’t really flaws in my book.
Carets- How big do you want? … Or more importantly how big does she expect? As I said above, a marquis cut diamond looks as big as a solitaire cut twice its size, but most women (I think) prefer the soliraire cut. At least I’ve seen more of them in the stores than other cuts.
Knowing what most retail places spend just on the store and location, I suspect that you could find a better deal online, but standard disclaimers apply about finding reputable places and knowing who you buy from.
If you have time, there is one way to get the perfect ring for cheap…
Go to a diamond wholesaler and buy the perfect diamond. Then go to a jewelry maker and design the perfect ring. The cost of ring and stone seperately will be about a quarter the cost of one premade, off-the-shelf.