Reed Switch Hookup Guide
The circuit set up for this example is about as easy as can be. First bend both legs of the switch to point perpendicularly away from the body of the switch, so they form a "U" shape.
Insert the reed switch into the breadboard. Then use jumper wires to connect one end of the switch to ground and the other end to the Arduino's D2 pin. Here's an example circuit:
That's all it takes! We'll use the Arduino's internal pull-up resistor on pin 2 to bias the switch high. When the switch closes, it will connect pin 2 directly to ground, and it should read low.