Just go to Tiffany & Co. Even if you do not buy from there they are the best for helping you understand what that diamond is all about, at least to me they are.
With color D E and F are colorless diamonds which tend to be your more costly diamond. G H I are almost colorless and the most common diamond bought. When you get into J through Z they are more yellow and can be very very expensive but for my taste the D E and F are amazing. They shine from across the room. I would rather have a one carrot D than a 2 carrot H.
The cut is not what everyone thinks, it's not about shape it is more about the symmetry of a diamond which makes all the difference in the world.
Clarity begins with IF which is flawless and rare very expensive. FI is almost flawless, then their is VVs1 and VVs2 which is a beautiful diamond and the most common bought by someone looking to spend a little bit but not break the bank is Vs1 and Vs2 then it goes into your Sl1 and Sl2 which has flaws but you would have to look very hard. Clarity is what makes all the light shine through amazing.
Carot is the weight of the diamond which like I said before I would rather have a small BEAUTIFUL diamond than a big cheaper diamond. But that is up to what you think your girl would like.
In today's world it is very common for the guy to ask a girl to marry him and then they pick out the ring together, but it's not as romantic. Surprise is everything with an engagement. I would get a ring that is simple and has a beautiful diamond, this way when you design the wedding ring she can either just go with a band if she is simple like me, or she can take your diamond and put it into her dream ring, which ever girl has.
Sorry for the long post but I hope I helped you a bit! Nothing like love, without it have more than half the music out there would be.
Good luck and don't break the bank, that is not the way to start out a life together!
(But spend it if you got it haha)