please write a number what maximum rpm do you want?
3000 rpm?
200 rpm?
10 rpm?
what is the maximum-speed as rpm that you need the stepper-motor- axle to rotate
or alternatively if you can write the number of the gear-ratio
what rpm do you want at the output-shaft of the gear-train
How many steps per revolution does your stepper-motor have?
Best thing would be to post the exact type of your stepper-motor and the datasheet of the stepper-motor
at least a link to where you bought the stepper-motor