You need at least a double pole relay so you can switch both the positive and negative lines with one relay. Your common output poles on the relay will go to your motor, and then wire the Normally Closed (NC) pole on one to positive and the NC pole on the other to negative. Then wire the Normally open (NO) pole on the firs tto negative and the NO pole on the other to positive.
Lester you might want to consider something like this motor controller from Hobby Engineering which is a simple kit and allows you to switch motor directions on fly. You don't say how much current your motor draws but this controller can handle a few amps.