You need to take off the cassette.

To do this properly, you need a chain whip and the cassette lockring tool. If you go to Performance and buy their Spin Doctor brand, you can have both for less than $15.

If you're gonna get serious about this you should just get the tools anyway, plus at some point doen the line it'll be good to take the cassette off to give it and your rear hub a thorough cleaning.