It is believed that the earth started to rotate because it was formed (as well as the rest of the solar system) within a cloud of hydrogen that collapsed down ((4.54 billion years ago) from its mutual gravity. As it collapsed, the cloud began to spin and continues to spin to date to conserve its angular momentum. Angular momentum is simply the fact that things tend to rotate.
I know you might wonder that there how come there is no force continually “pushing” the earth to keep it moving, and why does it not slow down. Actually, if something is moving, it doesn’t require force to keep moving, it requires force to stop! Otherwise, it will slow down and eventually stop i.e. “Objects in motion tend to stay in motion. Objects at rest tend to stay at rest.”
Again, thanks Princess. Keep your questions coming!