How do you calculate the Youngs modulus of a material?

To find the Young's modulus of steel or any other material you require a plot of it's deformation response to loading. Specifically it's axial stress vs axial strain.

From this you need to find the gradient of the straight line portion of the curve where the material is behaving elastically and obeying Hooke's law. This is essentially stress / strain and gives you Young's modulus.