Also, I am thirteen years old. (Mandatory mention of age for robot's challenge)
Step 1: Materials
A motor. I took one out of a broken Nerf Barricade.
A TIP31 transistor. $0.20 http://www.taydaelectronics.com/tip31c-tip31-npn-transistor-3a-100v.html
A 2N3906 transistor or similar PNP transistor. $0.02 http://www.taydaelectronics.com/2n3906-general-propose-pnp-transistor.html
A battery holder. I used one that held 3 AA's. $0.14 http://www.taydaelectronics.com/aa-battery-holder-5.html
A SFH 314 phototransistor. $0.58 http://www.mouser.com/ProductDetail/OSRAM-Opto-Semiconductors/SFH-314/?qs=K5ta8V%252bWhtbR6gV8hNfmATd6ftDkb0eH0AduyQe8MSk%3d
Perf Board. I used one I had laying around, but you should use Radioshack's miniboard, which costs around $2.19. http://www.radioshack.com/product/index.jsp?productId=2104052
A laser pointer is recommended to activate the light switch.
The total cost came to around $5, with shipping.
Must be able to solder and read schematics