First, I'm going to assume that the 12-3 to the switch has a black, white, red and ground. If it does not, you cannot add a new switch and fixture. The box with the switch in it thus has a "hot" wire, permanently hot, a neutral and a ground. That's all you need to run a 12-2 from there to a porch light. Splice into the permanently hot wire, putting it to the second switch. A new 12-2 (black, white and ground) wire will need to be installed to the porch, with the black terminated on the new switch, the neutral and ground of the new wire spliced to the neutral and ground already in the box.