5V Relay and Arduino to control a 6v nerf gun
Relax, it will not be that hard. First of all, work out which pins are for the relay's coil. Then, have a look at this article: http://tronixstuff.wordpress.com/2010/04/20/getting-started-with-arduino-chapter-three/  :)

[edit] D'oh - Joby beat me by 10 seconds