Depends what your problem is.
If you have a problem with the design of the uniform or the PE kit:
Get your parents to write to the governors of the school, because they normally choose the uniform and they can change it. They should listen to any reasonable complaint.
If you think you got an unfair punishment for breaking the rules:
Then you need to talk to your tutor or a head of year/house. Maybe you had a genuine excuse or were just wearing what you normally wear and didn't realise it was a problem.