Hello Julia,

The prevailing consensus is that the Sun formed form the gravitational collapse of a large cloud of gas and dust known as the Solar Nebula. The cloud was probably about a light year across and rotating very slowly. As the cloud collapsed the central core got denser and hotter. Eventually, the core got so dense and hot that the particles flying around (mostly Hydrogen) collide with each other with such force and so frequently that they began to stick together. Two protons stick together to form deuterium, two deuterium nuclei stick together to form Helium-3, two Helium-3 nuclei stick together to form Helium -4. This process is called nuclear fusion and it converts four hydrogen nuclei (protons) into one helium nucleus. The helium nucleus has slightly less mass than the four hydrogen nuclei that went into making it. The missing mass gets converted into energy (E=mc^2) and this energy is eventually released as heat and light and the star is born.

CHeers - David