You'll need a chain whip and a cassette lockring tool to remove the cassette. If you remove the rear wheel and look at the cassette you'll see the lockring. It has splines which a lockring tool keys into. Use the whip to hold back on the cassette and turn the lockring tool counter-clockwise. Park and Shimano both make lockring tools. My favorite is the Shimano TL-HG16. It has a pin that slides into the axle (where the QR goes) and helps keep everything in line when removing or installing the lockring. You should be able to find one at your LBS. Happy wrenching!