Everyone else gave great advice, but let me give you something to consider. Don't buy one of the bigger models, you know the ones I mean? They're sort of cylinder shaped, not too big but in my opinion just about too big to be useful..I'd find you a pic if I had time.

I bought one of these the first time I'd ever used a vibe and had no luck with it, the vibration simply wasn't strong enough for me. Get her a small one, pocket rocket or something like that. Those things are great, I know