How to stop being paranoid about social situations?

Sometimes I think a conversation went well, other times I wonder if that individual is just being polite. I also can't tell the difference between a girl thinking I am a nice person, or if she's interested in me. I think a lot of people have pre-conceptions and their hard to shake off. I don't trust many people with my innermost thoughts, but I do think I'd feel better if I got it under control.