The one big secret about these parties, and social situations in general, is that everyone is self conscious and concerned about what everyone else thinks of them. The irony of that is that no one is really paying attention to you, and the only way that you would usually stand out to people is by doing something outrageously memorable (which usually means outrageously stupid). But in the grand scheme of things, even those things generally don't matter.
I completely understand not liking big grounds or loud environments. Again, many more people than you would think feel exactly the same way. The way to deal with it is just to do things and be in areas that you're most comfortable with, whether that means quiet areas, areas with fewer people, or both. If someone talks to you, talk back. There are almost always plentiful opportunities to start conversations in groups of teenagers like this, because (contrary to appearances) basically everyone at them is pretty new to this and doesn't know what they're doing with themselves. If you just keep in mind that everyone there is insecure, it'll help you to just go with the flow and speak with whoever happens to be around to speak to. If you're invited to do something, such as talk in a group or play some sort of game, go with it if you want, and politely decline if you don't (or, if you're not comfortable declining, just make some sort of excuse such as needing to go to the bathroom). Again, no one is going to notice if you don't do something, don't talk to someone, or don't want to participate in some sort of activity. Those things are simply not memorable. Equally, no one is going to remember when you leave either, so leave when you want to.
Basically, go along, ease yourself into the situation, and do what you feel comfortable with for as long as you want to do it. This is a new experience, so just go there with the aim of finding out what you enjoy, what you don't and how you feel in that sort of situation. As long as you don't get outrageously drunk and do something spectacularly silly in front of a large group of people (and it sounds like you're sensible enough to avoid that), no one will remember that you were even there in a couple of weeks, let alone what you did or didn't do.