I read a lot and listen a lot to people around me. I watch the news so I know what is going on in the world. Listen to the conversations around you. What are other people talking about?
Ask the other person questions. If you're at an event and you're mingling, ask - how did you get involved with this organization? Is this your first meeting? Many people like to talk about themselves, so just start listening and asking questions. It's always awkward at first, but will smooth out with practice. Good luck.