Good question, and definitely possible to overcome. When I was around your age, a little younger, I was painfully shy, and people often mistook my silence as disapproval (and often didn't!). I still tend to be reserved in strange company, but can easily turn it on and off as necessary...and I'm sure that if I could get past it, you can too.
I don't know anything about the paraliminals Alex recommended, but one thing you can do to start with that won't cost you anything is to make a point of greeting people you meet with a *little* more effervescence than they are accustomed to, and that you are used to offering. For instance, if a cashier greets you with a token and disinterested hello, instead of just saying "Hi" back, say, with enthusiasm, "Hey, how ya doing?" When a waiter or waitress comes to your table, say "Hey, how's your day going?"
Practice assuming a more open posture when you walk or sit. Instead of having your head down, shoulders drawn in like you're waiting to be struck, force yourself to keep your head up...and maybe take up a little more room than necessary. (If you were a guy, I'd advise you to maintain eye contact with people for a second or two longer than necessary, but if you do this to guys, they might get the wrong idea--it doesn't take much for us to get the wrong idea.) Once this is easy, give yourself something a little more challenging to do...extend your comments a little. Ask people questions in an assertive and friendly manner. Do everything in baby steps.
Paul McKenna has a book out called "Change Your Life in Seven Days," and it has a good exercise in it in which you visualize your "authentic self"--the person you believe yourself to be, and want to be--as clearly and colorfully as possible and then step into that persona, until you feel, viscerally, the qualities of that self in you...and do it every day. I recommend reading that book for a full description. Maybe your library has it.
And, of course, realize that people aren't naturally disposed to think you are a foolish, laughable figure of fun. Unless you beat small animals and pass gas in elevators, people will be predisposed to think well of you, and those few who don't will certainly have their own problems that give them their churlish disposition. You have as much right to take up space as they do, and as much right to act as though you know it.
