How do you keep up a conversation?

Not always, but usually, being a little random can help keep a conversation going. There's an infinite amount of things you can talk to someone about, why not get creative about your questions and/or answers. Keep it light and lively.

It sounds like you are an introvert. Introverts have a hard time keeping up small-talk conversations and prefer to have deeper conversations. They don't see the point in small-talk about the weather or pop culture or the news, they would rather talk about the meaning of the universe and such. If this sounds like you, then don't be afraid to ask the deeper bigger questions.

Talk about things you have in common. If you don't have that many things in common then you can go into the 'story telling stage'. The story telling stage is where you tell stories about your life and then you ask the other person about their stories and so forth. But it really depends on who it is. Personally I think you can only use this move on a possible love interest.