This probably belongs in Teen to Teen advice, but it's still a good question.
First of all, not all silence needs to be filled. Have you ever heard the phrase "Silence is Golden". It can be true.
However, if you feel the need to fill the silence, ask about something the other person is interested in. It gets him/her talking, doing most of the talking, and it reflects well on you as it makes you seem like a person who is interested in others.
I've found that it does no good to force yourself to talk about something. Whenever I do that, I just end up talking about something they don't really want to talk about either. Find something you're both legitimately concerned about, and would actually like to discuss, then you'll have a good conversation.