How do I calculate the speed of the International Space Station moving in a circular orbit around the Earth?

Just equate the centripetal force and gravitational force acting on the ISS and find v.

P. S. Google the radius and mass of the Earth, the distance to the ISS from the ground, the mass of the ISS and the constant G. This is all you need to find the speed of the ISS moving in a circular orbit aroung the Earth.