help with Sensors

can any one explane to me how Sensors work with detecting like a bullet or something hitting it

You can use llSensorRepeat() but as that page notes they are rather inefficient and rather heavyweight (laggy) if you are trying to detect fast moving things like bullets.

Far better is to use the collision_start event which you don't even need to arrange a sensor for, it will simply be called if you write an event handler for it. The example in the documentation is a fine place to start.

You may also find llVolumeDetect() or llCollisionFilter() of use, but mess around with collision_start a bit and then you should be able to ask a more specific question.