How do I wire a dimmer switch to replace my existing old switch? Dimmer has with 1 green wire and 2 black wires . My existing light switch has 1 black and 1 white.
The white wire to the switch should be the hot feed if it is wired according to code. Connect the white wire to the top black wire on your dimmer and connect the black wire from the cable to the other black wire. Connect the green to the bare wire on the cable (and to the box if it is metal.)
Make sure your light fixtures are dimmable and the dimmer is the right one for the type of lighting you have.
What you have at your switch box is called a switch loop. The feed should be brought to the switch on the white and the black is the switched wire.
With your basic dimmer connect the white to one wire and the black to the other. It does not matter which.
Green/bare is ground and should be connected.