How would I control that, get it to turn round, stop where I wanted it to, etc
Your turntable and thus the stepper motor must have a 'home' position i.e., a starting point. Finding that point would have to be done every time the system, specifically the microcontroller, is started; easy to do in the Setup function of your sketch. From that point its simply a matter of telling the device how many steps to take and in which direction. That motor as 4096 steps per revolution. Your sketch must keep track of where the motor's shaft is currently at in order to be able to tell it how many steps and which direction you want it to go to a new position. As mentioned before, others have written libraries for this device that will make it not necessary for you to write the code to control the motor. You can simply plug in the values to attain the position you desire.
My 'home position sensor' of choice is an opto-interupter since that is a very accurate way of defining a position; think of how your desktop printer is able to position those tiny dots so accurately.