Staggered size with a step says that it's a freewheel you have rather than a freehub and cassete. You can either post some close up pictures of what you have and we'll identify it for you or there's no lack of pictures floating around that will aid you in identifying if this is a freehub or freewheel setup.

Assuming it is freewheel for now then you don't replace just the gears. You get a freewheel removal tool and useing a big solidly mounted vise or some other trick you unscrew it. The new freewheel comes as a screw on unit with all new freewheeling mech and gearing.