What work have you done so far? You need to choose a processor that you want to use. Then think about the sensing circuitry, Hop
@hevans1944 gave you an idea in his post above. Once you have done this we can fill in the gaps for you if you have forgotten anything.
You could have ago at drawing a circuit diagram unless you already have one,there are plenty of free circuit diagram packages out there. I can recommend DIP-TRACE but you may already have one. You will need to also supply details of the sensor and any pictures of the setup you have. Additional information like max RPM and any power requirements like current consumption and power supply voltages.
Tell us more about your project, your introduction was very short.
Thanks
Adam