How do I replace my rear wheel? My rear wheel was stolen this morning and I am unsure about how to go about getting  a new one without wasting too much time and money (I really want to start biking to work again!). My bike is really old and I'm not sure what parts I need to measure to figure out which type of wheel to get.

It's a Diamondback Apex adventure series that looks exactly like this picture.  

I'm completely new to bikes and don't know what the parts are called. It looks like the gear part was left over. Can anybody tell me where to start, and what to measure?
It should be easy to replace your wheel. The wheel is a 26" (this is a standard mountain bike size wheel), and you will also need to match it to the right cassette. from looking at your bike, its likely a 7 or 8 speed cassette. Your local bike shop should be able to tell what size cassette you need based on your rear deraileur, which wasn't stolen. With wheel, tire and cassette you are probably looking at a $100+ repair if you buy it all new. If there is a local bike co-op in your area that can help you with used parts and labor then you can probably do this for $30-50.