Turn off the electricity at the main breaker.
Go to the nearest outlet and unscrew the faceplate. With the faceplate off, you will see two screws: one on the top and one on the bottom of the outlet. Unscrew these, and pull the outlet out from the wall.
Run your electrical wire through the wall to the outlet you just unscrewed. Strip about 1 inch off the ends of the electrical wire, and screw onto the outlet screws. Make sure you are matching the wire color: white with white and black with black.
Screw the outlet back in. From where you started the wire in the wall, run it to where your new outlet will be.
With a hammer, nail your conduit box to the joist your garage door opener is on, securing it tightly. Put the wire through the conduit box.
Strip about 1 inch off the ends of the wire and screw onto your new outlet, using the screws on the outlet. Again, make sure to match the wire to the correct screw. The outlet will be labeled "white" and "black" for the wire colors. Screw the outlet to the conduit box and put on the new faceplate.
Plug in your garage opener and turn the electricity back on.