How can I separate a light switch to power two different devices?
This is a reasonable project if you have some experience with electrical wiring. You will have to take a look at the existing wiring, get the appropriate new parts, and replace the existing box and switches.

First, I'll assume that your current configuration is as such: supply power comes into your switch box. Wires leading to both your fan/light and your exterior light also run to this switch box. In your switch box there are 3 grounds (bare wire, all connected, and also connected to the switch and to the box if the box is metal), 3 neutrals (white, all connected) and 3 hots (black; supply is on one end of the switch, light/fan and exterior light blacks are on the other end).

That's the most likely configuration, but you should shut off power and check out the wiring situation before you do anything else. If that's right, you'll want to do the following:


Get a 2-gang box to replace your existing 1-gang box.
Get a new switch.
Remove your existing box and install the 2-gang box in its place.
Run your supply hot wire to both switches.
Connect the light/fan hot wire to one switch and the exterior light to the other.
Join all neutrals and ground wires.


You also mentioned wanting to add a fan control. For this you will need: a fan control dimmer, a 3-gang box, and a second hot conductor out to your fan/light fixture. If this were new wiring, you'd run a 14/3 with ground cable, which will have a white, black, and a red wire (and bare ground wire). In the switch box, you'd run the supply black wire to a switch (for the light) and to your fan dimmer (for the fan). Then you'd wire black to the light switch and red to the fan switch. On the fan/light fixture end, you'd wire black to supply the light and red to supply the fan.

In your existing situation, you'd need to add a wire to supply the fan separately from the the light. The typical ways to do this would be either replace the existing run of 14/2 cable with 14/3, or add a new length of 14/2 dedicated to the fan and keep the existing for light only. This choice is mainly one of doing whatever is practical given your construction options and materials on hand; either is OK.

Whatever you do, keep safe. Use a non-contact voltage test to make sure nothing you touch is live, and if you're uncertain, get a friend with more experience or hire an electrician to check your work.