Go sit where other people who seem interesting to you are. You don't even have to talk, unless of course one of them speaks to you.

If you sit next to someone you think is a neat person, just talk to them. It doesn't have to be about anything in particular; it can just be something totally random or pointless, like "Geeze, I hope we don't get much homework today." Or something.

Starting a conversation might be a bit tough if you're really shy, but if you can do it you can at least get to know some people a little better.