How stars are formed?

To know the answer of this question first we will see inside a star. What stars are made of? How they produce energy? How they live?
Stars are made of mostly hydrogen and helium. Hydrogen is the most light element having one proton and one neutron in its nucleus and one electron around it. Helium in the star is formed in star by fusing two hydrogen atoms together and releasing some amount of energy that can be calculated with Einsteins mass energy equivalence formula, the most famous equation of the world

E=mc²
here m represent mass which is the difference between the mass of helium nucleus and two hydrogen nucleus, c is the speed of light which is 3 X 10 ⁸ meter per second.
In this way stars produce energy and continuously consume hydrogen. So what made stars to shine is hydrogen. All this hydrogen with witch star is made of once very oddly distributed in some regions. And with some amount of luck the hydrogen atoms come together and forms molecular cloud, sometimes the molecular cloud is the part of nebula so it does contain some amount of heavier elements too. A gravitational instability occurs in the molecular cloud sometimes with no apparent reason and sometimes triggered by outside activities like supernova explosions, this makes the core of this molecular cloud matter very dense and hot under the effect of gravity, so that the Jeans Limit is reached.
What is Jeans Limit?
Well a British physicist James Jeans calculated that under certain density and temperature conditions when the mass of star crosses a critical mass, it triggers the runaway collapse, which is the phenomena when all the matter around starts to collapses into the core due to gravitation. This gravitational collapse lasts for ten to fifteen years.
The new born stars emits a jet of unused and by produced gas.
When the hydrogen atoms in the core are sufficiently hot for the nuclear fusion the star produces energy and also builds an outward pressure. This outward pressure, and the inward pressure because of the gravity, remains in equilibrium for the star, until it stats dying. How they die is a new and interesting topic I will catch later.