How do I know which wire/prong is positive and which one is negative?

Middle to the middle. Check. The other two are "hot" 120 volts each. They can go on either side it does not matter. Once you get the middle one you are set.

Is the middle wire red or is it white usually? Thanks

White. White for neutral and Black and Red are hot.

Green is ground. In the main breaker box hots are on one side and neutrals and grounds are on the other. On a 4 prong cord the additional green wire is a back up in case something happens to the white wire.