How do i open my lock when i don't remember the combo?

Start guessing.

Most cheap locks aren't that sensitive, so you don't have to try every number. The sort used in school lockers will probably open if you're within 3 of the correct value, so you can try every fifth number. Of course, if it's a school locker, you could just go to the office and get the combination again; they record this sort of thing.