This is an interesting and deep question. The scientists who study this question are called cosmologists. Most cosmologists agree that our universe was born about twelve billion years ago in an event they call the big bang. While the science of the big bang is pretty well understood, it is difficult to explain, and very weird stuff. Some things that are important to know are that it resulted in a lot of energy and a lot of hydrogen gas that was spread out in lumps in the early expanding universe. It also apparently produced even more ‘dark matter’ and ‘dark energy,’ but no one knows what they are yet. Maybe you will be able to help us with that some day!
After a few billion years, some of the lumps of hydrogen gas settled down and gravity pulled the hydrogen in the lumps closer and tighter together. This made the temperature rise; heating the gas so hot that nuclear fusion began. In nuclear fusion, atoms combine to make larger atoms. Fusion releases a lot of energy, raising the temperature of the gas balls, and creating new and heavier elements. These balls are what we call stars.
The big bang made mostly hydrogen and I think a little bit of helium. These are the two lightest elements. Stars can cook all the elements up to elements as heavy as iron out of hydrogen and helium, but as the amount of iron increases, it slows down the action of the star. Eventually the star runs out of energy and collapses. The collapse of the star can set off another huge reaction that blows it to smithereens and creates even more elements, up to the weight of uranium. These are scattered throughout the space where the star had been.
Our sun was born after this had already happened. It started as a lump of hydrogen, and condensed to become a star. The star had enough gravity to attract some of the remains of the earlier star, which condensed first into a ring or rings like those around Saturn, and then those condensed into the planets. Earth and all of its materials including you and me are the product of hydrogen from the big bang and everything else is from stardust. Earth condensed about four and a half billion years ago, and our sun will keep on being like a sun for Earth for at least another four and a half billion years.
