I’m not going to give a full physics lesson — that would take too long. Instead, I’ll assume you already know some basic stuff. So, I will start with an object moving around a planet (it could be any planet) a height h above the surface. Note: for Low Earth Orbit, this height is about 400 kilometer.
Here is a diagram (not to scale).
Illustration: Rhett Allain. An object orbiting a planet. It could be Earth, who am I to say?
The key here is that in order for the object (the red ball) to move in a circular orbit at THAT altitude, it must have some specific velocity. An object moving in a circle as an acceleration vector pointed towards the center of the circle. This is the centripetal acceleration and it has a magnitude of: