I have a ceiling light in the garage. I opened up the switch and there are only three wires; black, white, and ground. I'm assuming this means that the power is coming from the light. I would like to add an outside light and and have it controlled with a second switch powered from the first. I have replaced the electrical box with a two gang box but the wiring has me confused. Any help would be appreciated. (There is a receptacle in the garage and a hallway light on the same circuit. )

Assuming you want locate the switch in the same box as the present switch, you must either replace the 2-conductor cable between the light and the switch box with 3-conductor cable or you must add another 2-conductor cable. Then from the existing light you run a 2-conductor cable to the new light.

You could also run a 2-conductor cable from the outlet to the switch, then to the new light location. You could also run a 2-conductor cable from the outlet to the switch and a 2-conductor cable from the outlet to the new light location.

Thanks for the reply. Your answer just confirmed what I was already thinking - a job for a licensed electrician.