I have to warn you that, depending on your state, county, and local laws, you may need to hire a licensed electrician to legally replace light switches/power receptacles or any structural wiring additions to those components.
Let's begin by replacing a light switch.
The first thing you want to do is identify the breaker box circuit being used in the switch box. It makes it easier if you or someone else has already pre-labeled your breaker panel. If you don't know which switch box belongs to each breaker, you can try the flip and test routine. Flip a breaker off and see if the switch still works. If it does, then go back and try another breaker. Be sure that everything in that switch box is actually turned off before you go digging around in it. You can use a voltage detector (found at any hardware store) to double-check that the box is dead, but check it on a live circuit first so you know that it's working.
Other than the voltage detector, I didn't use any specialized tools.
Here we have a "before" shot of the light switch.
You'll want to read the directions so you know about the wiring beforehand, but it makes it easier if the original wiring was done properly. You'll notice that I have a red wire connected to a black wire in the picture. Red and black don't mean the same thing in AC wiring as they do in DC wiring. Generally, the black is the load wire and the red is the runner (which takes the load to the next device in the chain for a three-way switch), but they are easily interchangeable (and sometimes need to be) to get a load to the right switch. Green is the ground wire, but is often just a bare copper wire. White is neutral (hot) and is sometimes in the back of the switchbox. In many older homes, it's not in the switchbox at all. When the neutral wire is not in the switchbox then it was run directly to the light fixture; that's likely a sign that your home's electric could stand to be re-wired, and you may want to talk to a licensed electrical contractor to at least have them take a look.
I'm also changing out the other switch in the box to the “Decora” style to match the Insteon switch.
If possible, you should try to do your work such that only one conductor is exposed at a time. The rest should be in wire nuts or connected to terminals.
Since I was changing to Decora style anyway and had to change coverplate, I spent the extra money for the plates that hide the screws.
Hidden screw plates leave a much cleaner look.