About 5 billion years ago—8 billion years after the origin of the Universe--a star in one arm of the Milky Way galaxy collapsed in a supernova, forming a cloud of hot, interstellar debris.
Under the influence of gravity, this material flattened into a rotating disk, and in this rotating disk, particles collided, forming planetismals. This process can be compared to the formation of dust bunnies under the bed, although the force that attracts the dust particles is electrostatic attraction rather than gravity-driven collisions.
Planetismals collided with each other, eventually, about four and half billion years ago, forming the planets of our solar system, including Earth.
For more information on this solar nebular hypothesis of the origin of the solar system see this link or this one.
Illustration is from this site.
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