How to be confident?

Gear the conversation to a topic you know

If you commonly find yourself feeling insecure in social situations, and as though you have nothing to contribute to conversations, there's a simple trick to fix that: Talk about what you know.

Obviously you don't need to walk up and just start aimlessly talking about it, just strike up a casual conversation with someone about something they've said. Once you're talking to them about what they said,relate what it is you know to what it is they said. (It's not as hard as it sounds.)There are infinite ways to relate topics to each other. Just find the overlap. Practice it on friends first if you don't feel confident doing it with strangers, every step you take towards practising this will increase your confidence in social situations exponentially.