Why does my circuit breaker keep tripping?
First choice:  Call a licensed electrician.
Second choice: Find all the outlets and lights which are affected by the breaker.  Start with the one closest to the panel.  Open them all up and visually inspect.  If nothing is seen, turn on the breaker with everything opened up - does it still trip. Sometimes movement in the boxes will re-position wires and remove a short.
If you still have a problem, start removing sections from the circuit - i.e., disconnect them from the line. Start in the middle - check breaker if no more tripping your problem is down line from that point.  Get the idea?