The Earth has a long history that stretches back over 4.6 billion years ago when bodies of rock began to collide together and form larger rocks in the process. The Earth was the product of these rock collisions, and when the Earth was formed into a planet it was extremely hot for several reasons.
Internal pressure helped to raise temperatures from the gravitational contact of these large rock collisions. There were also large amounts of radioactive elements early in the history of our planet that released heat through radioactive decay. The other reason is tied into the first, with multiple rocks and asteroids were colliding with Earth and these large bodies traveled at tremendous speeds through space which created immense heat on impact with Earth.
