If you have a data sheet for the motor, it shows the correct phase progression to get the motor to step. Some motors will function with unipolar drive where current only flows in one direction through the active windings (connect 12V to the common, and then ground one or two windings at a time). The drive transistors only switch to ground.
Other motors require bipolar drive, where current flows alternately one direction, not at all, or the other direction. Bipolar motors require push-pull drive transistors.