Stormy is right. Don't do any actual whitening treatments or your teeth will be spotty when you get your braces off.
Strawberries are supposed to be natural tooth whiteners.
Also, if you floss often and brush your tongue, it will prevent bacteria that discolor teeth.
If you drink coffee, soda, tea, etc. Always at least rinse your mouth right after if you are unable to brush at the time.
Good luck!