In Minecraft I need to make a piston door that opens when a plate is stepped on, it is normally the other way around. Does anyone know how? A video would be helpful but not required.
Simply put in a NOT Gate. A not gate is a redstone current running into a block which has a redstone torch attached to it (as shown in the first screenshot).
Here is a good example that uses a NOT gate.
If you already have the door working (but inverted) just connect the pressure plate to the pistons using a NOT gate (a torch on a block) to invert the signal and therefore the door.
Put a NOT gate on your line from the pressure plate to the pistons.
Put a "not gate" from the Pistons to the pressure plates (if you're using them). A not gate is where you make a redstone trail up to a redstone torch on a 1x1 wall. Then on the other side of the block, you make another redstone trail to directly underneath the pressure plate/whatever else you're using and (optional) one more trail on the right side of the block (redstone torch view) to directly underneath your other pressure plate.