These links may help get you started
Stepper Motor Basics
Simple Stepper Code
If you need more help than post a link to the datasheet for your stepper motor and tell us what stepper motor driver you are using.
Your requirement suggests that you need to set the step count to 50 and FWD when the button changes from not-pushed to pushed and then count down the number of steps. And again set it to 50 and REV when it changes from pushed to not-pushed.
...R