How is there a fixed destiny if we have free will?

Well, as far as I am concerned we have a fixed destiny. But I also know that it can be changed by dua. But if one doesn't make any dua then? I suppose it will remain unchanged. If so then where id our free will. Then will do what we were ment to do. But then there will be no free will. So, please can anyone help me by telling how these two lines up?