How did the Earth form and become spherical?
The Earth's Shape : About five billion years ago the solar system was forming from gases and dust floating in space. Gravity, the force which pulls all objects in the universe together, caused some of these gases and dust to come together to form the sun and all the other planets. The Earth formed first as a hot ball of lava from the coalescence of these gases and the impacts of meteors. The animation below gives a visual representation of this process, known as the Nebular Hypothesis. The Nebular Hypothesis, which is the view accepted by most scientists today, is fueled by gravity and tends to form spherical bodies which rotate.
The Earth is not a perfect mathematical sphere, so we refer to it as a spheroid. The exact term which describes the shape of the Earth is oblate spheroid, meaning that it is a slightly flattened, imperfect sphere.
In summary, the Earth formed about 4.5 to 5 billion years ago when the solar system was coming together from space debris and gases. The general name for gravitational forces causing planets and stars to form is coalescence, and specifically, the Nebular Hypothesis is the scientific term used to describe the process which forms planets and stars.