Lighting, Light Fixtures, Ceiling and Exhaust Fans - Add ceiling fan where no fixture exists

Hi,
I would like to add a new ceiling fan where no existing fixture or power exists. My question is how to tap into existing power.

3 sets of 14/2 Romex in junction box
All 3 whites joined
All 3 grounds joined
Black wires:
1 to bottom of Off/On switch
2nd to top of Off/On switch then joined to 3rd
Switch controls outlet on wall below (1 set of 14/2 in box)
1 set runs down wall to another outlet not controlled by switch
1 set is source from switch/junction on opposite side of wall
Opposite side of wall has same looking setup
If I disconnect wires from opposite wall, power to this switch and side of wall is lost.

Can I tap into power from this switch? I would like to replace the existing single gang box and replace with a dual gang, keeping this setup and adding a light/fan switch.
Thanks.

I just answered this same question for someone else...

Yes, you can tap this. The book you read on home wiring should have told you how. You did read a book didn;t you? Attempting to do electrical work without a clue can get you killed.

The two black wires together are the hot wires. The single black wire takes switched power to the receptacle. The white wires are, of course, the neutrals.

Looks like this is already wired with 12/2 even though it is only a 15 amp circuit. I purchased 14/3 to run to the fan/light but would assume it is best to use 12/3?
Thanks