How does one gain the confidence of talking to guys?

I think I have a huge problem. I am a 17 year old girl who is afraid to talk to guys because I always run out of what to talk about. Most of my friends are girls and I really appreciate them. I also have a little bit of boy friends in school who I am very comfortable talking to and all that. But when it comes to festivals and hanging around down-town and meeting new people that´s where I always seem to shutdown and lock myself in my shell. I start sweating and I fell like I have BO and it just drives me nuts. Sometimes I say ‘hi’ to strangers and that´s pretty much it… we never seem to say anything else but I really wish I did.
What can I do?