Luke,
Are you asking how to turn on the DC power to a PIC when Light shines on a photo diode? If so, there is no programming required, just a DC switching circuit which is activated by the photo diode.
Is the light on as long as you want the PIC to be running, or do you want the PIC to start with just a momentary flash of light, and then keep going?
What happens when the light goes off? Should the PIC go into a state where it should remember what it was doing prior to the light going off?