How do I use an existing outlet to run a new line?
If you can illustrate this, it would be a huge help.
In my garage, I have a garage door opener on the ceiling and I would like to get electric from there and run a new outlet to one side (concrete walls exist).

The task is simple enough, what may be more complicated is peicing it all together.

In principle, all you need to do (assuming your garage opener plugs into a regular wall outlet, that happens to be on the ceiling) is remove the cover, connect your new wire to the unused terminals on the outlet. White to silver and black to brass and bare copper (or green) to the ground screw (normally painted green).

The on the other end of the cable, repeat.

However you need to provide more information.

Such as is your excisting outlet box recessed into the ceiling, or does it stick out? (this can make a big difference in the amount of time it takes and cost to compete the project, for your sake I hope the outlet is not recessed, then you can just run conduit)

Will you want to run romex or conduit?

More importantly, are you sure that the garage door outlet is at the end of the line as far as that circuit goes. If not, then you will need a juntion box, which opens a whole new can of worms.

But that's the basic of how it goes. Some of the more expirenced onces here can quote some actual codes for you to go by. I just wanted to give you a general idea.