Speed Detector with a Buzzer+LED+LCD

hi…
i am a totally freshie in Arduino… im going to make speed detector by using magnetic reed, and i need the arduino to make the buzzer sound after a speed limit is reach.

what I need right now is the coding for that to work.

what does the reed switch do? use gps

the reed swtich is to detect the speed of a spinning wheel, (gps???)