After you get some momentum, practice putting your back foot into place on the back of the board, just ahead of the tail curl, around the trucks. Get your balance and ride, bending your knees some to keep your balance.
Mongo foot means that you feel more comfortable pushing off with your lead foot, and riding with your back foot. Some people see it as acceptable, but it will hinder you later and it's an awkward motion to swing your front foot around. If you find yourself pushing mongo, try switching from regular to goofy or vice-versa.