The earth is made up of many layers (the crust is the outside part we live on, the mantle is the middle and the core is at the center). The core is the most dense part.

The core and mantle used to be one continuous lava (which we call magma), but over millions of years, the core separated from the mantle.

When you put dust or dirt in a glass of water, it starts off my mixing, but eventually the heaviest pieces sink to the bottom. This is what happened to the core and mantle over millions of years-- the heaviest elements separated and accumulated together to form the core. This is why the core is more dense than the mantle.

Why does it orbit
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Earth has a core because all of the iron in it melted and sank to the center, because iron is denser than rock. Earth orbits the sun because of speed and gravity: it is moving too fast going sideways to simply fall down, but is not moving so fast to fly away from the sun, so basically it falls around the sun in a circle.