Based on bikepedia, looks like you need a 700c wheel http://www.bikepedia.com/quickbike/B...town&Type=bike

Low end bike, 13-34 7-speed, almost certainly a freewheel NOT a cassette. Should be a reasonably cheap replacement available at a bike shop, maybe $50 for a new wheel. Take the bike/wheel in with you so you get the right one.

The wheel is unrelated to the cassette/freewheel and chain. While the cassette/freewheel/chain may be worn out, they have nothing to do with the rear wheel.