Like Daniel said, try and talk to the people you sit by in class. Even if its just to ask them something about the work you're doing. Just get yourself comfortable speaking to them in general.
How about smiling at people? Don't walk around the corridors, head down, ignoring everyone that passes. If you recognise people from your classes, smile at them, say hello. You never know, they might approach you and try to engage in conversation.
Some advice that i was given, no less than 30 minutes ago, actually, just be yourself. Forget what people might be thinking about you. Don't try and tone down your responses just because you think people will judge you. Say what you feel (unless its going to offend someone).
Just take things one step at a time. If you make friends with just one person, say by the end of the month then that's brilliant. They're then likely to introduce you to their friends, and soon you'll know a few more people. And once you've got some confidence, you can go out and talk to more people.