I'm replacing a dimmer switch with a new one - its the sole switch for the light. The new dimmer has 2 black wires. The box however has 3 wires going into it (none going out); a red (120v), a black and a white. I assume I have to use the red, but do I use the black or the white with it? Thanks in advance!
If it's the only switch, & you have a single cable leading to the box with red, black, white your (non-smart) dimmer or switch goes to black and red, unless someone got way off base with standard wiring. The white should be neutral in that case, and it's "modern" switch loop wiring which is required to support smart switches.
It's possible that you have a three-way setup you are unaware of, which could change the role of the white wire.