As your self confidence builds you will come out of your shell. Don't act crazy as others have said as going from one extreme to the other won't solve your problems. Just look at people when they speak to you & when you speak to them. If they smile relax & just let the person inside shine through in a relaxed state. Nervousness will subside I'm sure as you have more faith in yourself. A friend of mine who was shy started acting in plays. She said she was able to be someone besides herself while acting & it took the pressure off her while she was in that role. Give it a go. It might work for you.
"A lot of people have this time problem. But sometimes you just have to do it, just talk to them and not think about their reactions. If they get mad or be rude to you, be aggressive back to them and you will feel more confident because they're the ones that started something in the first place. Most of the time people aren't going to criticize you right in front of you and people always want to talk instead of sitting doing hw or reading a book. And also the "you only live once" concept helps people. Best answer me if this helps you." ~ Krazed Logic, Yahoo! Answers
To break shyness you need to step out of your comfort zone! Challenge your self to socialize through out the day. Even if it's the cashier at the gas station...Ask them how their day is going! Find people who have similar interest as you do so it's easier to find a common ground! Use good eye contact and smile when you talk! Think positive! don't put your self down for anything You think you did wrong or stupid. Most the time people are shy because they often battle with themselves. I hope that makes sense and is helpful for you.
Think of this: When you're going to talk to someone or they are coming to you, what are thinking about you? They think they are going to talk to someone normal, I mean, with no shyness whatsoever. So, it's you that make it tough for yourself. (I'm not criticizing, but making you aware) As others said, just relax and don't think about what you should do. You WILL automatically say or do what you should. Everyone knows. Imagine yourself in situations where you're talking to someone and you have no anxiety.