Pretty much everything I have to say has already been said. Eso nailed it on the head.
If you're not a very social person and plan on attending a party loaded with people you don't know, you're only shot at having a good time would be to get piss drunk. Otherwise you'll end up sitting in the corner watching everyone else have the fun, as Havocan mentioned.
I've been to maybe 8 or 9 real parties in my life, none of which were really that big... maybe 30 to 40 people. The only thing that got me out the door was the fact that in all cases I had at least 3 friends with me, and knew at least a few more people at the party itself. At about half of them I got drunk, and the other half I stayed sober... and every single time I stayed sober I ended up sitting on the couch the whole night like a douche bag, not knowing what to do. And as for the times I got drunk... actually had a lot of fun even though I generally hate alcohol.
If I was you, I'd first try to get a feel for what kind of people are gonna be at this party. See if it's your type of crowd or not.
I remember in highschool, somehow me and 2 friends ended up getting invited to a friend of a friend's party. The host was some stuck up chick from another school. Parents were out of town, so she was throwing a huge ass party for the 'popular' kids of another school.
I had a bad feeling about it, since we didn't really know anyone going. But my outgoing friend insisted relentlessly. So I finally agreed to go, expecting the worse of course..
We didn't even make it through the door. We knocked, they opened and then proceed to just laugh at us and then slammed the door in our faces. Even a full bottle of vodka wouldn't have saved me from the humiliation I felt that night haha =/