This, fortunately, is a fairly easy problem to fix. It means that you swapped the two outside wires on the potentiometer. Most volume-related problems, in fact, relate to the wiring of the potentiometer; for other similar problems, check the wiring on the potentiometer.
To fix this problem, swap the outside two wires on the potentiometer. You may need to re-solder the connections; to do this, melt the solder out of the way with a soldering iron, pull out the wires and swap them, and re-melt the solder back into place, adding a little solder if need be.
Closeup of potentiometer in guitar.