I’m one of the worst people you could ask about this because I have the same problem. Social anxiety is one of the main reasons I’m seeing a therapist right now. I’m like a mega-introvert.
The best I can offer is a piece of advice I picked up from Gala Darling: pick out a confidence role model (e.g. Amanda Palmer, Shane from The L Word, your friend who is always out clubbing) and ask yourself, “What would [that person] do in this situation?” and then go do it.
I also find it helpful to dress myself in a way that clearly shows off some aspect of my personality or is a good icebreaker in some way (T-shirt that says “I love vibrators,” anyone?!) and then go up to someone and compliment them on something they’re wearing.
Honestly, this is like (as Robin Williams would say) a leper being the judge of a beauty contest, so I’m not sure why I’m even offering advice on this, haha.