Hello   There are plenty of guides...if you will on the Internet as far as different sexual positions and ways to achieve orgasm.  But honestly I am thinking that perhaps the nervousness comes from putting too much pressure on orgasm.  Orgasm is just a small part of intimacy.  Enjoy the process of getting close.  And when it comes to pleasing your partner the best thing you can do is to ask her.  Ask her what feels good and what she likes.  Everyone is different and you can find some supposedly wonderful sexual technique but your girlfriend might not like it.  So talk to her.   I wish you the best of luck.  Thank you for your question.