I am replacing an existing toggle switch with a new dimmer. In the existing box, there are 2 white wires maretted together and not attached to the existing switch. There are 2 black wires screwed to the top screw of the existing switch and one red wire screwed to the bottom of the existing switch. I can see a copper wire at the back of the box but the end is not accessible. The new dimmer has 2 black wires and one green. Being blonde and electrically challenged I have not idea how to hook the dimmer to the existing wires.

Looks like you have a three way switch layout but have a single pole dimmer switch. You will need a double pole dimmer switch. Follow the diagram on the switch or write back with further queries.

Connect the two black wires in the box to one of the black wires from the switch using a wire nut. Connect the red wire from the box to the other black wire of the switch. Remove the bare wire from the screw in the box. Connect a short piece of bare or green wire to the screw. Connect the two bare wires and the green from the switch together with a wire nut.