Firstly, does your first year count for anything? A lot of university courses don't count first year results in final degree classification, and if this is the case then don't worry TOO much about what you've missed. You'll have to pass the year, so try and do as much as possible to avoid missing that target, but don't spend hours and hours on things to get top marks if it doesn't matter. Just do your best on all assignments going forward.
Definitely get into your classes, get involved with what's going on, try to talk to people about work if you don't know what else to say.
My best friend from uni just came and sat next to me during a lecture when we were asked to discuss in groups, and I'm very glad she did now! The worst that can happen is that it turns out you don't get on with certain people and then you can move to sit elsewhere. I was still talking to new people all the time in third year, it's never too late.