Depends on the girl and on you as well. The two most common positions are doggie style (easy entry, the girl can really go to town on your ass) and missionary, usually placing a pillow under your butt to raise it up off the bed and with your legs up in the air, especially your feet back by your head, which really opens things up. I think this is most common, especially for a first time encounter because it offers easy entry and you get to see her and enjoy her female charms while she hammer your virgin ass. But there are any number of positions, some of which I would never have believed possible under I tried them.
Doggie style with her straddling you on her feet rather than on her knees is nice. Or side-to-side, either from the front or the back. Or try laying on the edge of the bed, with your ass off the bed, face down, and let her enter you standing.
A favorite of mine is to ride the girl on top, her on her back and you facing her and lowering yourself onto her cock. Careful, though: if she's big or thick, and you drop down too fast, it can hurt! (This happened to me with a well-endowed Latina girl once, ouch!)
I love to wrap my legs around the girl as she's fucking me missionary style, playing with her tits or kissing, locking her into me with my legs around her hips. Or same position, but her sitting more straight up and beating me off as she fucks me.
One of my favorites (you need a very limber girl and have to be in good shape yourself) is sitting facing one another, with her inside you. I've done this with Venus, and it is very intense, as you can really explore one another's body while feeling her inside. Then slowly lay back so your loins are locked and your heads at 180 degrees from one another. You don't get any real motion this way, but it feels amazing.
Of course, there are more variations! Have fun, play around. Oh, and while some call "top" "active" and "bottom" "passive" (in Portuguese, "activa" and "passiva"), the more active you are as a bottom the more intense the sex will be. Get into the rhythm and move with her!