I currently have a ceiling light that has the power at the light. It is switched with a single pole and both white and black wires are connected to the switch. The white is hot. I take it there is no neutral in this case. I want to gang up a second switch to power a new light in a nearby closet. How do I jump power to the second switch?
You will have to run power from a convenient location where it is available to either the new switch location or the new light location. Then you will have to run wiring between the new switch and the new light.
With this being a new installation, if you run the power source to the new light location, you should run a 3 conductor wire with GND between the light and the new switch.
You will not be able to simply tap off at the existing switch to power the new circuit. From what you describe there is power at the existing light location and so that may be a convenient tap point to power the new light circuit.
An alternative if you are able to replace the existing wiring between the current light and switch is to replace the existing 2 conductor wire with new 3 conductor with GND. The black and white of this new cable would bring power from the light box down to the existing switch location. The switch would connect to the black HOT wire and to the alternate colored wire (most likely red) and supply power to the current light which would connect between the red and white at the light box. This scheme would then allow a tap from the existing switch box for power to the new light.