Resistive touch screens are pretty easy to interface with, There are four nodes, you apply a voltage to two specific nodes and read from either one of the remaining two which forms a voltage dividier giving you an analog voltage of the position on screen. You do this twice energizing different nodes and reading from the third to get either the X or Y axis. Capacitvie touch screens are much more complicated and I don't think it's practical to do it with a PIC directly, you're likley going to need a specific Ic for it.