There is not enough information about your stepper motor, so let's assume some: let's use stepper motor with 1.8 degree per step (very popular step motor), and we will drive without any microsteps.
To accomplish the task you would need:
- stepper motor,
- stepper motor driver (like EasyDriver or some kind of Pololu drivers)
- power supply. To avoid interference I would suggest powering up your Arduino with PC (through USB port) and additional, powerful PSU for the motor/motor driver only.
Now, you would need to provide at least two signals from Arduino to stepper driver (what I am saying here is example of EasyDriver). The signals named DIR and STEP. DIR selects the direction of the motor to spin. Depending of the stepper motor to driver PCB connections you may have clockwise spin when signal is in one state and counterclockwise when DIR is in opposite state. STEP accepts pulses to move the step motor. Driver moves the stepper motor when STEP input sees raising edge. One raising edge equals to one step = 1.8 degree (again, this apply when you do not use microstepping). To spin motor 180 degrees you need 100 steps.