The first thing your instructor will show you is how to adjust your seat & mirrors correctly, and then go on to show you what the main controls of the car are.  

 

Adjust your seat and steering column properly. Fix their height and distance so that you can sit comfortably. You need to move your seat so that you can get the left pedal (the clutch) all the way down to the floor without stretching for it and you can grip the steering wheel without having to lean forward.

 

Remember you can move the seat backwards and forwards, you can also alter the angle of the backrest and possibly lower or raise the height of the seat as well.  Remember that you might not know exactly what the best position is at first but don't be afraid to tell your instructor you need to stop and have another go.  Eventually, you'll know what feels good.