Possible Duplicate:
ok so my character shoots fireballs, and i want it that when my character runs into a certain item it changes color and shoots something else.(Like in MegaMan X; example: Runs into a Blue Sphere = shoots iceballs.)
These things are explained in the tutorials, since almost every game needs something like this.
The concepts you need to look for are Colliders, which are used as Triggers, and functions like OnTriggerEnter().