How do I make her cum?

You should find out where her G spot is and experiment with what feels good on her G spot. Once you figure that out, play with her clit and hit her G spot at the same time. Also anticipation is key. begin pleasing her slowly and lightly and then building up will help her body orgasm. The guy that was able to completely pleasure me had this down and I would have crazy orgasms. He told me he read an article on it from askmen. You should read it because it WORKS!

Has she ever had an orgasm before? Has she ever given herself an orgasm? If not, she needs to do some self exploring to figure out what she likes and what works then communicate that to you.

If that's not the issue then she needs to let you know what works for her and what doesn't. Get her to communicate what she likes you doing and what she wants you to do. She knows her body better than you and has to help guide you.