1. Avoid chains like Hyperplex said.
2. Find out what she likes.
3. In the event you do not want to do #2 first or it turns out that you are mistaken or that she just flat out changes her mind, find out what the return/exchange policy is and make sure it is in writing prior to purchase. Many places assume the guy will pick out something he thinks his future bride will like to surprise her, and that the two of them will then come back and jointly shop for something else as her eventual "forever" ring once the wedding date is closer.
This.
Also, try and find a diamond broker or gemologist jeweler in your area. Avoid any place that has track lighting. Track lighting is in EVERY mall jeweler because they make EVERY stone shine like crazy.
Here in Cincinnati there is a broker called Koch Jewelry. They're on like the 10th floor of an office building. They never advertise, yet have been in business for over 50 years. A friend told me about them when I got married the first time. I made an appointment and the gemologist took me back into their vault and gave me a lesson on diamonds that I'll never forget. He told me:
1. Know what you want to spend.
2. Buy for quality not carrat. My ex wife wanted a 2c+ princess cut. But that was to to be outside of my budget.
Then he showed me how most stores sell crap. He placed a stone in a setting and had me look at it under a microscope. It was beautiful. Then he dumped the stone on a white sheet of paper, and it was yellow as fuck. We looked at the loose stone under the scope and it was loaded with carbon and imperfections. The lesson...stones that are preset hide poor quality. Buy a loose stone you pick out and have it mounted in a setting.
We talked about the 4 c' s and I was blown away by what I learned.
Most bad stones are cut with a shallow cut, which allows the light to escape the bottom and sides, but since they're pre mounted they still sparkle, so we went with a stone that was Ideal cut, and it's brilliance was like a white hot fire, no matter where you were in the room when you looked at it. This cut meant it was a round diamond now.
As for Clarity I chose an IF stone, which means Internally Flawless. This is a very rare stone. It had no inclusions and minute blemishes, you'd need to be a gemologist with about a 20x microscope to see.
But anything at VVS or VS is still an excellent stone. Most of them are what are known as "eye clean" that means you'll never see anything bad with the naked eye.
It was about a two hour education that I've cherished ever since. We wound up with a nearly flawless stone that was 3/4 carrat. It was worth more than just about any of the larger stones her friends had, and she loved it.
I've bought jewelry at other places since, but I'll never buy a diamond anywhere else. In fact, I bought the second Mrs. Vox a flawless emerald cut diamond there. It cost me almost 40 grand. About a year after we were married, she was with a girlfriend at a local chain jewelry store who was buying her husband a watch. The jeweler saw my wife's ring and asked if he could take a closer look. He looked at it under his loupe and then after telling her how gorgeous it was asked if he could see it under the scope that was on the counter. So she took it off and in front of her he looked at it and said, "It's not every day I see an 80,000 dollar ring come walking in here."
Buy quality, buy a stone and have it set, and avoid chains.
Like the gemologist told me all those years ago..."Nearly everyone buys quantity. If you have the money, I can get you the Star of Africa if you want it. But if you don't have that kind of money, why would you want your wife to wear a big piece of junk? If it's smaller, but an heirloom quality stone that will appreciate over generations, that's a much better way to go".
EDIT.
I'll also leave you with this nugget from a priest the first time I got married. He said, " Couples spend an enormous amount of time, effort, money, and energy planning and preparing for the FIRST day of their marriage; and not nearly enough of all of that on the REST of the days of their marriage".
Congrats and good luck.