Socialising problem.........?

i get intimidated by people who are pretty and confident with themselves because i don't believe i am either of those things. I am worried about what these people think of me because i see them as perfect and because they are perfect i think they will think i do everything wrong. i know what i do wrong but i don't do anything about it because i don't know how to change it. When i'm with a group of people who are having a conversation i leave it up to everyone else to say something. I don't know why i do this but it might be because i can't think of anything to say or maybe its because i just can't be bothered to say anything. But i feel really selfish because when i do say something everyone has something to say about it but when someone else has something to say i don't say anything back. I do try to say something but when i think too hard i can't think at all. When i haven't said anything for a while im thinking so much about how people must think im the most boring person ever that i ignore the conversation everyone is having and then i can't include myself in it. I need to stop being so selfish and to be more normal and socialise like everyone else. How do i do thiss??