And being a witness doesn't always mean that you are running around telling everyone about Jesus (though, it is important to tell people why Christ came and died. Who would want their best friends to spend eternity in hell?!). It means that you are keeping yourself pure and just, and reading your bible and praying which produce a joy that unbelievers don't have. It means that you are optimistic, that you set your standards high; that you take care of the temple that the Lord has given to you (aka your body). When you do that, other people will notice.
So go let your light shine!