Stride up to class in confidence, and show them you have what to talk about. Sounds like you're really shy, and you shouldn't be! I'm shy too, but well not at that level. Like you said you don't have a lot of time to be a kid, so just flow with it. It's not like you're dealing with something that could make you lose anything. What's the worse that could happen?
Even if you find one good friend you can talk to about stuff, that'd be great. It'll help you open up towards others as well.

Conversation topics could be anything. A great way to open up a good conversation is watch a show everyone's watching and talk about it. If it's comedy, you can redo some jokes you saw on that show and remind everyone the laughter! If it's a reality, you could always talk about who you think would be the next to get kicked out.

And it's a life rule, there's always someone in your class who's just the type to be your new best friend.

After that you could just go hang out outside, go to parties, etc. :3