She probably wants to experiment with you, she's being a little nervous and scared of it, that's why being "straight acting". When she comes over, just tell her what you just told us. Tell her that you want a serious relationship with her and you want to stay with her for a long time. If she says yes, congratulations. If she says no, she wants to use you for sex, then she isn't for you.
But anyways, if this is your first time having sex with another woman, just experiment. Watch a lot of lesbian porn and do what they do.
