Do you need a rear wheel because your current rim got damaged (out of round, etc), or are you looking to change hubs or otherwise upgrade the wheel? In general, you'll need a 622mm wheel (often called 700c) in a similar width as what you have now (probably somewhere between 15mm and 20mm). The hub will likely need to be setup for 135mm spacing, and it needs to have the correct freehub for the number of speeds you have in the back right now. Is your current setup a 9-speed?