The tacho will only read correctly when the fan is operating at full speed, not if you are using PWM to reduce the speed. But if it is the hot end fan that you want to monitor, you are probably running that at full speed already.

The firmware is already set up to read and display the RPM of one fan. Connect the tacho wire through a diode (1N4148 or similar should work) to pin PB6 on the expansion connector. The cathode of the diode (the end with the stripe) goes to the tacho wire.