First of all stop worrying about what people are going to think of you. That is the biggest mistake you could make. Every extrovert you meet will always insist that they never worry about what people are going to think of them, because they know that the other people are worried about themselves and if they're not then they're just enjoying the conversation. Seriously it takes a lot of thought to cultivate a fascinating story, a funny joke or a smart quip. People can't focus on other people and their own participation at the same time. So don't worry about other people's opinion of you because they're more then likely concerned about their own self-image or they are so busy in the conversation they don't have time to think of you like that.
Secondly, just practise, practise, practise. Appreciate that regardless of what you say, people would much rather hear you say something then nothing.
I appreciate that people are all born differently. Some of us are born with the quick wit, self-confidence and on-the-spot intelligence to function well socially. Some of us aren't. It's the curse of the laws of probability (and genetics too I guess). But all of the above skills are very much learnable. If you give it a bit of time it won't be long before you reach that point you always dreamed of.