How can I be more outgoing?

I can never relax and freely talk to people, not even online. I always worry way too much about what they think about me and the things I say or do, even though I know they probably don't think anything at all. I need months to just start talking to someone casually, but not many people have the patience for that...And I never initiate conversations because I'm afraid they will tell me to **** off or something...And when people are friendly to me, I am still pretty withdrawn, because I'm not sure how friendly and open am I supposed to be. I see people getting close all the time after just a day, and online, even though I might me a member of a site for months, someone new will become much closer to regs and stuff...and I sometimes can have a good conversation with someone, but that's just once and that's it...I have no idea why...

I have tried to be more outgoing sometimes, and it worked, but I don't know...