"Socially deprived" as you could say?

I'm 15, a sopre in HS. My dad is really strict because of my bipolar (not proven) stepmom which actually seems like a sister that constantly argues with me. Anyway, I feel so "socially deprived" because I hang out w/ my best friend maybe every other weekend *the rule* and I focus on staying best friends with her than anything else since we've been friends forever. I play soccer, so I get a lot of social life out of that. But tonight, for instance, I went to her band picnic and I felt so awkward. I was just sitting there and everyone was laughing and joking around, walking around. And I felt so uncomfortable, like self-conscious.. and I never feel that way too much. Is there a way I could get myself to open up and act more like myself like I would act in front of my best friend. How do I seem more interesting, I guess you could say?? I just never really have much to say when I feel uncomfortable around different people, what should I do/act like? Please give me some advice!!