If the locker is secured by an external lock (like a padlock), the most inexpensive method of opening it is to cut it off with a pair of bolt cutters. If the locker is secured by a built-in lock, contact the locker owner, manufacturer or a locksmith.
A locksmith can open just about any external padlock, but paying for the locksmith's services is more expensive than simply cutting the lock off and buying a new one. Padlocks can be cut with bolt cutters, but a disc lock may require an angle grinder. If the locker has a security bar or an internal lock built into the locker, first try contacting the business or school to see if it has a way to open the lock.