Girl invited me round to her houseWatch
And try to be not outcome dependent. IE whatever happens, as long as it doesn't result in death or permanent injury or disfiguration, should be seen as a totally fine and cool outcome by you.
If either of you are under-age, on no account should you have any form of sex.
If you don't find her attractive enough, physically or compatability wise, you should aim to be a really good platonic friend for her - assuming she's a decent enough person. If she's not a nice person, you should aim to break off spending time with her.
If you're scared because you want to give her the most fantastic time, but you're not confident that you'll be able to do that, then you have an OK attitude. Just do the best you can for her. This could be a great learning experience for you.
Try to focus on her as much as you can. Don't focus on yourself. Her mood, emotions, thoughts, feelings, not yours.