I have a single switch and an outlet in a box in the bathroom. I am installing a bath fan and would like to operate the light and fan separately. I have a double switch. The trouble I am having is the switch only has one black, one white, and ground. The outlet has one black, one white and one ground. They are both hooked up separately. How do I incorporate another line one black, one white and one ground from the fan, Everywhere I read there should be one more black to have power coming in?
What I get from your description of your bathroom circuit is you have both a wall receptacle and a light (presumably in the ceiling) with the light controlled by a wall switch in a single gang box. You want to add a vent fan and control it by a separate switch placed in a 2-gang box which would replace the existing single gang box.
In order to do this you will have to get both an "always hot" and a neutral to the exhaust fan and then have a "switch loop" to the new switch to control the fan. From your description of the wiring in the current box containing the light switch the white wire in this box is not a "neutral", but rather it is a "switched hot" going to the light. If this white wire is attached to one side of the switch, it is not a neutral.
There will be both a neutral and an always hot in both the box for the light and in the receptacle box. From one of these boxes you would connect a new cable to the new fan box to provide a neutral and a hot there. Then you would run another cable from the fan box to the new switch which would control the fan.
It might be easier to simply add another 1-gang box for the new switch for the fan rather than replace the existing 1-gang box with a 2-gang box, but I believe the usual way to do this is to have a 2-gang box.