Shortly after the birth of our universe in the big bang, space was filled with energy and hydrogen atoms. (I am skipping over the question of dark matter and dark energy because I don't understand them.) Hydrogen atoms are the smallest and simplest atoms. As the Universe expanded, it cooled, and the hydrogen gas atoms formed clouds.
These clouds had gravity, and the gravity pulled the atoms closer and closer together. When atoms move apart, as in the expanding universe, they cool down. When they move together, as in these hydrogen clouds, they heat up. After a long time, they can get so close together and so hot that two hydrogen atoms smash together hard enough to fuse, or join together to make a helium atom. This releases energy, and more atoms fuse. This is the birth of a star, and it happened in places all over the universe.
Eventually the helium atoms start fusing with hydrogen atoms and with other helium atoms, and the star starts making heavier and bigger atoms like carbon, nitrogen and oxygen. This system works until our star starts making iron atoms, which are the limit for most stars. Iron is a very stable element, and difficult to fuse. Iron also poisons the star process, and when enough iron builds up, the star collapses. As the star falls in on itself, it again heats up, sometimes enough to blow itself to pieces.
This explosion, a supernova, has enough energy to create the rest of the elements that we know in nature, and blasts this stuff out all over the local part of space. This stuff is what Earth is made of. In time a new star was born, our sun. Earth is mostly iron and nickel metals, which are dense heavy materials, and at first it was like a huge pile of dust and rubble floating through space, orbiting the new star. Early in its life, Earth was smashed by something or several things huge, releasing enough energy to melt it. Iron and nickel sank towards the center, and the lighter, less dense elements floated to the surface. These materials today make up our atmosphere, oceans, earth's crust, and you and I.
