How awesome for you WK, Congratulations.
Have you seen any of her jewelry, I am sure you have.... The shopping will help as you can listen to her talk about what she finds attractive about different cuts, color, clarity, carat, the 4 C's. Then come back with what you feel she is attracted to, or what she says she likes. It is important to know the order she places the 4 C's, as she views them before you pick her ring, is carat more important that clarity (at what inclusion does she say uh, no), is cut more for depth than width (sparkle will vary with cut depth and width, so does the illusion of size), what color range is she comfortable with, and if she leans toward modern design or something traditional or even antique. How fun, I loved ring shopping aaawwwww.
Check out here to get the basics of what to ask her or just to listen for.
About the 4Cs
and
Tutorials - The 4 Cs of Diamond Grading
How exciting!