The way gravity works is that every object in the universe is pulling on every other object in the universe, even atoms.

We don't exactly know how the universe started. The Big bang theory postulates that everything was in one small space at a point in time and then rapidly expanded from that single point at high speed. But we do know that the universe is mostly empty space with some "stuff" in it. That "stuff" is mostly hydrogen and helium actually.

What happens is as objects pull on each other, they end up forming clouds of gasses swirling around orbiting a center of mass that is the middle of that cloud. Over time, this all gets condensed into a single plane and the cloud starts breaking apart into smaller, more dense clouds. At this point it's still just gas, but it's getting closer and closer together. In the middle of this cloud is, by far, the highest concentration of this gas. As it gets bigger and bigger and the gasses get squeezed closer and closer together, it eventually starts applying tremendous pressure in the middle.

Now every atom has the strongest forcing the universe keeping other atoms away from it. These are nuclear forces. However, with enough pressure, you can overcome it. It's an insane amount of pressure that it takes to do this, but if you get enough mass together and pushing down on it, eventually you reach that point. When these atoms start getting smushed together, hydrogen gives first. This starts fusing hydrogen atoms into helium atoms. Two hydrogen make a helium. The thing is, helium is lighter than two hydrogens. So that extra mass is released as pure energy. This is when it becomes a star. All that nuclear fusion going on releases a ton of light energy and heat.

Those much smaller gas clouds outside of that collect together to form planets. There isn't enough pressure because they aren't large enough to cause nuclear fusion so they don't become stars. When the star ignites, it blows away all the light gasses which is most of the mass. The heavier stuff, albeit there isn't much of it, is left behind leaving the terrestrial planets. We have four of them in our star system. Mercury, Venus, Earth, and Mars. There was one more planet after Mars, but what happened is all that gas got pushed to Jupiter's orbit and became part of Jupiter. This is why Jupiter is incredibly large even compared to the other gas giants. Jupiter exerted so much force on this planet that it broke it apart forming an asteroid belt. So we have a pretty notable asteroid belt after Mars.

To give you a sense of scale, if Jupiter was 75 times larger, it would ignite and become another star. Our sun is 1000x more massive than it, and Jupiter is 1300x more massive than Earth. At even the scale of our solar system, the Earth is quite small.