A good start would be asking her what feels good the most and focus on that. Don't focus so hard it starts to hurt.
You can find sites, books, magazines almost anything can give you tips and ideas on how to give your girlfriend an orgasm.
The easiest way to give her an orgasm, generally speaking, would be with her clitoris. G-spot orgasms happen, but from what I've heard and know they're kind of tricky.
Many tips:
1: Don't stress out and do not be in hurry to *** or orgasm.
2: Rub her clitoris.
3: Give her oral sex.
easiest way by rubbing her clit also during coitus...