Stop being a bitch?
I mean I really don't know how to answer that. There's no special method or trick to being nice, you just have to be nice. It's hard sometimes.
edit/ I don't mean to imply that you ARE a bitch, just wasn't really sure how to phrase that.
I usually have to take a few deep breaths to deal with people that are annoying me. What's in my head is a lot lot lot bitchier than what I actually say. Stress can make it worse. I never really got along with girls either, some people just don't. If you're more rational than emotional the cattiness gets tiring.
And if the breaths don't work I take it out on my sparring partner or a punching bag later. They don't talk back and they can't make your life suck. :)
It sounds like you are already getting there, because WANTING to be a nice person and trying to do something about it are a great beginning. Just make sure you are doing this for yourself, not to gain their approval. If you do it for them it will never stick.
The first step I take in being consciously kinder: If it would hurt to have someone do it to me, I don't do it. The best thing I can do when I start to feel catty or bitchy is to shut my face. Closing my mouth enables me to judge what comes out of it as appropriate or not. And don't be afraid to suddenly stop being a bitch and apologize. You may feel stupid for suddenly changing direction, but in the end you will find out the person you were being mean to will respect you more for it.
Tell your friends to tell you when you're being a bitch, and make sure not to get angry at them for it. I find it helps me be conscious if someone lets me know what I'm doing.
I hate drama too, but to be honest I just lifted my eyes from all the current 'crap' and remembered that when I got out of high school all their childish drama would seem like just that- childish drama. Be the actual 'cool' kid and walk right by it.
Let them know you have a hard time expressing yourself, and instead of taking it out on them tell them, "You know what? Lets go play some basketball/kick some ass on Halo/ skateboard and blow off some steam. I bet that would make me feel a ton better, and then we can talk about this later when we've had time to think."
It will help, and you can make new friends that way.
Well, you could think of this one of two ways... either you can forget anyone who hates you and be yourself and surround yourself with people who like you for who you are, or you can pick out your faults and try to change them... not to make anyone else happy, but to make you happy. If you feel like you're too bitchy, tone it down a bit. It'll be hard, but if you really want to change yourself, then you'll just have to bite your tongue and be a calmer person. But if you're only concerned about the people around you... fuck them. Find people who don't mind your outward and bitchy personality, and screw anyone else who gets in your way.

Girls your age can be very catty and if you go around talking about how pretty you are, yeah, I can see why some girls may be less-then-nice to you. Try being humble?
You could start by not thinking so highly of yourself. o;
Maybe you think you're drop dead gorgeous, but that could all be in your head. Before focusing on what's needed to make yourself popular, perhaps you should get to know yourself better. P:
Regardless what you'd call yourself, lookswise, you seem to focus far too much on that... the outer shell of who you are... when you should worry about what kind of person you are and how you treat people.
you need to figure out what you really want. do you want to be nice and make more friends... or do you want to 'express' yourself at the drop of a hat?
it seems the problem you have is the idea that keeping your mouth shut is somehow an injustice to you -- which is rather odd. you'll have to learn what/when to say things when you get older, so why not study up on it now?
this doesn't look bratty or immature at all.
