1. Have a jeweller you can trust. There are anecdotes about people taking your finished ring to "resize" at the back of their shop and swapping your diamond for something else in the process. I went with a wholesaler well known to our family in Toronto (I'm Canadian and used to go back every year). I ended up with a great diamond with excellent finishing at an awesome price with a separate independent assessment for insurance purposes.
2. Read up and educate ++++. I highly recommend the forums at pricescope.com. Learn about grading systems. GIA is baseline minimum. However, there are GIA "triple excellent" stones that do not perform well to your eyes mainly because GIA cut grading is determined by measurements only. Mine came with an AGS grading with laser light analysis. At the end of the day, I picked a very well-performing stone, both on paper and to my eye.
3. Precious metals are personal preference. Anecdotally, if you can get platinum, I would choose it over white gold as white gold has a lot more nickel than I thought it would. Platinum is 95% pure platinum, correct me if I am wrong. To each his own.
4. Don't get lost in choosing the stone. You are doing this for your future life partner. All the stuff mentioned before. Ditto.