Well for starters, sex changes things. In most cases it can bring two people closer together, but you need to have a solid base first.
Secondly, I would say that you shouldn’t expect sex to be “perfect” your first time. Things almost always go wrong (even with experienced people). She will probably be just as nervous and self-conscious as you are, and will probably be more worried about her own performance than yours. I don’t think there is any secret to having good sex. The key is communication. Everyone likes different things, so the trick is learning what those things are. Pay attention to her body language with every touch and movement and encourage her to tell you what she likes. Assuming she knows you’re a virgin, she will probably be quite understanding throughout the process and will offer you guidance as you go along.
As for the size issue, I wouldn’t worry about it. The media builds it up to be a huge deal, but most girls I know prefer average size because it’s easier to work with. Too big can just be uncomfortable, and they say girls can only feel the first few inches anyway. It’s all about how you use it, and as long as you’re looking to please her, you shouldn’t have any issues. Remember, it’s a learning process so try different things and see what works. You’ll find your groove in no time.