How do I make a small sensor [...] when it detects too much light it sets an alarm off?
Many fire alarm systems include thermal detectors, as a quite effective means of detecting fire.
I don't know exactly what fire you wanna detect, the ones under the pot or those that may arise from forgetting your stew for too long (Which may sometimes only result in a glowing mass and black smoke).
In each case, a thermocouple bead could be a viable solution, as in any case, the temperature will rise rather quickly in case of a fire. Further, Thermocouple beads come in variants that easily cope with 1300°C.