Seems to me that douchebags have high self confidence.
Looks like the guy above me just said the same thing! But mainly, it's a confidence issue. I'm not sure where the line gets drawn though, does high self confidence make you a douchebag?
Luckily, since presumably we're all a bunch of nerds around here, we would go around classing ourselves as 'nice guys'.
Good! Groundwork done.
Confidence can be learned. I was really quite insular during school, but later found out that everybody else is pretty much just like me, and will definitely talk to someone who talks to them.
Top tips!
-Think of it like an rpg in reverse. It starts off difficult and gets easier as it goes along.
This is true of a lot of life and sucks, which is one of the reasons we play lots of games with a learning curve that's not retarded.
-Similarly, saying the opening hello to somebody you don't know is much, much, much harder than every other sentence you say after that. Just do it, you have nothing to lose!
-Make sure that you have a large and readily available supply of funny/interesting stories to tell people when you meet them.
If you're talking to girls, don't relate anything gaming related. You might luck out and get a hot gamer girl with a nice personality, but you probably won't.
If you don't have any non-gaming-related stories, go out and get some!
Embellishing kind of lame stories to make them better is definitely fine, just keep track of them so that "One time I went to my friend's house and he got totally smashed and tried to wear a plant on his head" doesn't turn into "OH GOD MY FRIEND GOT SO DRUNK LAST NIGHT AND HAD SEX WITH A FLOWERPOT".
A friend who will probably meet your new friend later.
-Nobody judges you nearly as much as you'd think, and not nearly as much as they judge themselves. Your average person is so worried about what other people think of them that they don't have time to judge other people. Use this to your advantage! Talk to everyone for those crucial +1 internal confidence points that will make everything much easier as it goes along.
-Sit next to the girls who are sitting on their own. Everyone likes somebody to talk to, and they're much the same as anyone else. Of course, if they turn out to be shallow and vapid, use them or leave.
You can usually judge how friendly they'll be by whether they're smiling or not when they say hi back.
-If you're a virgin, leave your morality at the door. The first time doesn't matter (although if it does, bonus!). Make sure to keep your standards on your person at all times, although they will decay rapidly while drunk.
-Flirt, but don't be sleazy. Actually, don't be too nice either, that's even worse. Otherwise it'll be that long, fast track straight to the friend zone that I'm sure we've all experienced.
Learn from the douchebags, but don't be a douchebag. Girls aren't innocent little flowers any more, after all.
-Finally, facebook stalking is an accepted and necessary part of modern life. Don't feel bad for finding out her age and whether she's single or not!
tl;dr
Talk to everybody, meet new people, and always have a story to tell.