Installed new plug and when disposal was turned on, a loud pop and spark came from the switch. I'm trying to find the hot wire and the outlet is dead. When the disposal switch is on, both the red and black wire are hot.

Do you mean you are replacing a outlet under the sink for the disposal.
Have you done anything at the switch. Tell us what wires you have in the outlet box and how you connected them.

Red and black? Is this a 240V? Are you sure red and black are hot?

Brian
Right now I'm thinking two circuits sharing a neutral. Have to wait and see.

Yea, could be a duplex or maybe connected red and black on the switch?

I may be misreading, but it sounds like the dishwasher and disposal are sharing a receptacle, and the disposal is connected to the switch, while the dishwasher is separate. You may want to look at the old outlet, there is a small metal breakout piece between the brass (hot) screws that may or may not be removed on the old outlet, if it has been removed you will need to remove this breakout piece on the new receptacle, (but NOT on the neutral side). But that applies only if this is your problem. You may also need to check your switch wiring.

Thanks guys but I figured it out. I guess when the built the house they wired the outlet to two different switches. The pop/spark thing I think was because I didn't break the (hot) tab. All is working and we just have to remember to keep the DW switch on. :-)