The Earth and other planets were loose mass that got escaped from the sun which was spinning at a very high speed. As it gets thrown out from the spinning body it got the same speed of the main body from which it got ripped off and also started revolving. Any mass with loose particles in it will get attracted towards the center of the spinning due to a force called centrifugal force. Even now earth is not absolutely smooth ball like thing, it is highly irregular mass, but because of the spinning looks like a ball. Any object for that matter spinning around its own axis will look circular if it is flat, look like a ball if it is not flat. You can try out this experiment. Take a fist size of soft clay and run over a stick in the center, spin it like a top and see how looks like. If the same thing is mechanically made to spin at greater speeds, the can see the irregular shape slowly changing to a spherical shape. Our earth was originally of some kind of big mass of irregular shape, which went on spinning and revolving for billions of years. The effect is what you see our earth as spherical shaped.