I have a whirlpool duet dryer that is around 6 or 7 years old. It has started squeaking and thumping and leaving black marks on clothes. I have never repaired a dryer before so I'm not really sure where to start.

What you need to do is to move the dryer and take it apart. When you pull it out from the wall and unplug it you will be able to see what you need to do in order to take it off. It is not that hard to do. Then remove top and front. Have dryer on its back to make it easier to get the drum out. The first thing you want to do is to clean out everything very well which means to remove parts that move air and wash them out with soap and water, including door and blower fan. Just vacuum out motor. Some dryers have a rear drum bearing and that can be causing your problem. You always grease bearings in a dryer with white lithium grease, available at any appliance or auto parts store. If the bearing is not scored then you can clean it well and re-grease it put it back in, but if scored, which it probably is, then get a new one as it is fairly cheap. You also want to get a belt for the dryer unless you want to do this again in a month or two. It is only about $5.00. Put it all back together and test the drum by rotating by hand to make sure you have it right because many errors can be made in putting it back together. If you have labeled all wires when you took it apart you will have no problems.
Look at these: https://dl.dropbox.com/u/8010163/Whirlpool%20Dryers.zip