That's a high-level question, and the answer may be a little bit difficult to understand because some concepts should be clear. So stars are made out of clouds of dust and hydrogen gas. These clouds are massive in size and take hundreds of thousands or millions of years to get converted into a star. So these clouds want to dissipate in the space, but gravity holds them together.
So a tug of war starts between gas pressure pushing out and gravity pulling in, and eventually, gravity wins with time, the materials crunches down into a disk that is the beginning of becoming of a star. As more and more gas particles are sucked into the centre of that disk, it becomes denser and denser and harder and harder, and at 18 million degrees hydrogen atoms fuse to form helium, and with a burst of nuclear energy, a star is born.