On most of them, you have to turn the lock to the "on" position. If you don't have a key that fits, you'll have to pick the lock. Fortunately they are easy to pick. Search YouTube for any number of videos; but the basic idea is to put a bit of turning force on the barrel, while you reach in with a small tool and lift each tumbler in turn so it lines up. The turning force holds the tumbler in place until you get them all lined up and it opens. As you lift each tumbler, you should feel a spot where it kind of clicks (which is where it lines up). If one tumbler doesn't click, do the others and then come back. May take several tries to get them all up in position, so just keep working back and forth.
Once it's in the on position, there should be a button or pin that can be accessed through the hole in the side and pull the barrel out the front.
Or of course you can just take the whole thing to a locksmith and ask him to make a key for it.