I disagree with the comments about forcing yourself into 'situations'.
Pushing yourself into a conversation with someone, where you could end up speechless, stumbling, fumble etc will ruin any confidence you have and make you even more shy.
Instead of forcing yourself into huge overpowering situations, slowly integrate yourself. Start off with small comments; Smile and say hello to people you work with or people you see often at school (people in your class,etc). Next time, maybe ask how they are. It'll build up your confidence because people will always reply to you if you say hello or ask how you are. Listen to conversations of the people near by you. When you feel more confident or if the topic is about something you know, try and make a comment. Say they're talking about a film you've seen, say something like you thought it was good.You don't have to say much. Just one comment. After that, it's just a case of building up from one liners to a few comments every now and then to fullblown conversations. Just take it gradually and slowly. It's all about building up confidence.

This is how I managed to integrate myself into my work place. At first, when I started, I only smiled and nodded, because I was too shy to say anything else. But eventually, I built up from saying hello to making a comment to eventually having conversations with people. It IS a lengthy process but it worked for me.
One thing though. As you are shy, you're probably be very quiet. So don't take it as a personal criticism if someone can't hear what you were saying. Just repeat it if they ask. I have a problem where I instantly go 'don't worry' if someone doesn't hear me, which actually ends up causing more fuss than I can really withstand.