It should have a yellow and gray wire already to your console, that will be your tach signal. If it doesn't it should be located on the bus bar on the side of the engine. You will also have to have a ground and a wire that comes on with the other lights in your instrument cluster. If you don't have any other gauges you can check and just source a wire that is hot when your switch is on for your light.
Then just buy an OMC tach or a suitable aftermarket gauge. Pay attention to the instructions on the tach especially if you go aftermarket. It will have a little dash pot on the back of it with settings something like 1-5. The instructions will tell what settings to set it for your corresponding make motor and #of cyls.