How fast can u run 400m? any tips?

very fast
That's pretty fast. What does your coach say about how you can improve?

You'd be smart to let your body mature a bit more..and keeping working hard...with weights, swimming pool resistance work in the shallow end, running intervals...and sprints. At 14, you don't want to burn out. Always allow your body ample time to heal so you won't get stale or push your body to the point of breaking down. At 14, you can do damage if you push too hard too soon.

Good luck and keep your passion for the sport.