Light Switch?

I have a question about my light switch. I want to put up an outside light next to my back door. Inside the door are two light switches, one goes to the hall light and the other doesn't turn on anything. When I opened the switch box there was a black and ground connected to the switch, the white wire was disconected with a cap. There were black, red, white, and ground going to the hall light switch. How do I connect a wire going from the light to the switch? And what kind of wire should I use? There are black, white and ground comming from the light fixture.
Thanks~

 

With out seeing, but as you describe it, sounds like the switch feeding the hall is a three way, with power and grounded (neutral) conductor at another location in the house. The other switch also has grounded (neutral) conductor at another location in the house. So, to use any wires from the hall switch can not be done, then the other switch you probaliy have a hot, but no neutral. Sounds like you have power in a light some where, and neutral never came down to switch. and with white caped off, Dont think I would trust the wire to be any good. Now to fix this you will need to run romex 2 wire (14awg for 15 amps, 12 awg for 20 amps) to some kind of power source, be it recpt. or breaker, or j box in attic with power in it. mach wire size to be the same as wires you find to hook to. cap off two existing wires feeding switch, and install new wire along with wire to new light, both in switch box, both 2 wire. you will then have two blacks, two whites , twoo grounds. one black to one side of switch, other black to other side of switch. two whites wire nut together push in back of box. two grounds wire nut together with pigtail hooking to switch green screw.
Hope this helps, post back if you need help, and let us know how the project turnes out.