The single pole light switch is the most common, one light switch controlling one or more light fixtures.
A 3-way light switch is where you can control one or more lights with two switches (common scenario at the top and bottom of stairs).
Where the word supply is used in the guides below, refers to the main wire supplying the electric to the switch.
To prepare the wires to be connected to the light switch strip the plastic off the ends of the wire a half inch down from the end.
In the first picture below you see three wires sticking out of the outlet box. One wire supplys electric from the breaker box, another wire feeds electricity to a outlet, and the other wire goes to the light fixture.
As shown below in the last picture the supply wire and the wire going to the outlet are connected to the same screw. Then the wire feeding the light fixture goes to the other screw.
First you take the black (hot) wire from the supply wire and insert it into one of the holes on the back of the switch (you can only use these holes if its 14 gauge wire, if its 12 gauge wire you have to use the screws on the side of the switch).
Take the black (hot) wire going to the light fixture and insert it in the other hole on the switch.
For the white (neutral) wire's take the supply wire and the wire going to the light, twist them together using a pair of pliers.Then use a wire nut to cap off exposed ends.
Twist the bare ground wires together leaving one wire about a few inches longer than the other, then connect the longer tail piece to the grounding screw on the light switch.