KE7GKP:
Start by measuring the peak voltage (at maximum RPM) with a volt meter. Then use a voltage divider (two resistors) to divide that peak voltage down to a voltage that is safe for the Arduino (i.e never greater than 5V).If you don't already have a digital multi-meter (for measuring volts, amps, ohms. etc.) you should have one if you are going to do any experimenting with electronic circuits. They can be found for as little as $5 if you shop carefully.
Ok and after that what do I do? I mean with the code and hookup?