Heres my thoughts. This is all from experience.
First of all, is she a virgin, or not?
If you rush things, then she will think that you are just after sex. Don't be afraid to be sexual, but don't just put your hand down her pants a minute after you start kissing her. Be a little slower, dare I say, romantic. Get her turned one first. Stroke the back of her neck when kissing her. Kiss her neck, they love that. There are plenty of places you can touch a woman, that aren't the tits or pvssy, that get them turned on.
Aim for small targets first, if you're getting head, handjobs etc, then you'll get there and at least your having some sort of fun. If you have nothing work towards these 1st.
Assuming that you this, then think about why she's resisting sex. Heres some common ones but there are others too.
1) POWER - A very common reason. She wants to make you wait to gain power in the relationship. As I've said regulary, a man should have power, if he doesn't then things go wrong.
2) She feels dirty, uncomfortable with her sexuality, perhaps for religious reasons. Make her realise that sex is perfectly natural and enjoyable act.
3) Shes underage. I do not encourage underage sex and suggest that you don't date underage girls in future.
If she isn't a virgin, then I'd make it clear, in a subtle way, that you can get sex elsewhere without too much effort. Mention other girls that you like, etc. It's more likely to be POWER than anything else here.
If shes a virgin, its more likely to be (2).
Do remember that if you bang virgins they have a habit of being fond of you for a rather long time.
I wouldn't wait longer than 3/4 weeks for a non-virgin and perhaps a little longer, I'd decided in my last relationship with a virgin to wait no longer than about 1.5 months, however I did wait 2 months simply because I knew she wanted it and it was just a case of finding a place to do it!