I'm going to split this article into two parts: one part on heating, the other on AC.
One important note before I get into this article: Be sure to check your local laws before working on your AC system if that's what you intend to do. Some places don't allow do-it-yourself-ers to service their own AC systems, and they also might prevent you from purchasing refrigerant. We'll talk more about this in the AC section of this article. For now, on with the show!
In general, HVAC problems can range from the simple to the very complex. Most HVAC systems are behind your dash, which means that if you need to get to those parts, you're going to have to remove it. This can be a daunting task, but with a bit of time and patience, you should be able to get through it.