Riding...

I'm 18 and I've only had sex with two people, my current boyfriend and I have been together two months and have become really sexually active lately. 
The first guy I had sex with.. We only did it missionary, but my current boyfriend and I are doing lots of different positions. 
I've been riding him lately and I don't think I'm doing it right. I have a lot of trouble moving around up there and it just feels awkward. Help? :(