Ok. Just to get this out there. This conversation is odd for me because you're 15, but then again i've been there and I want to help you feel a little less nervous.
First off, i'd advise the girlfriend over the "upstairs girl" at a house party. Regardless of what you see in movies. However, in general, if she's going upstairs with you, she likes you, so relax and hang out.
As for all the details, how long you'll last, etc., don't worry about it so much. My advise is as follows. She's as confused and nervous as you are. Probably more so. So, whatever happens, just act like that's what's supposed to happen. She'll like 1 minute of fun more than 10 minutes of being nervous and awkward. And you'll probably be ok for a second round shortly after anyway.
I'm 29 and had sex with a new girl for the first time last night. I only hung in there for a couple minutes, and that's with my practiced self control. You just have to expect the first time to be short and go with it.
As for precum. Just some fyi. Precum and premature ejaculation are not the same thing. I'll spare the details on precum. Just look it up. As for premature ej, it might happen. Its happened to me. It probably happens to most guys at some point. As dave chapelle says, "i don't even believe in that. If I cum, it was right on time!".
So, again, whatever happens, act like that's what's supposed to happen. Oh yes, and don't say "i'm sorry", etc. There's nothing wrong with what's going on. Just tell her it was great, she's really hot, etc. Then, if you can after a few minutes, go for round #2.