It can go to two places. You can do the following...
Plan A
1. Open up your distributor and remove the rotor and dust cover.
2. On your coil, if you look carefully, there are going to be two terminals, one that reads (-) Negative and (+) Positive.
3. Unscrew the wire thats on the negative terminal and add the Green Tach wire along with the existing wire thats already there.
4. Screw it all back together, dust cover, rotor, and cap, start that mofo up and you have yourself a RPM reading. Simple as that.
Plan B
1. If you have helms manual for your model engine, look up the stock TACH wire on your harness and splice it in. But both methods work the same and Plan A is easier.