how do black holes form?

A black hole is a highly dense substance in the universe,which has infinite gravitational pull that it can pull anything .Not even light can escape it's infinite gravity.
find more about a black hole here.
 so how do black holes form?
Through Quantum theory anything can be a black hole from light,atom to a star.
 A black hole made up of light or any form of energy is called a Kugelblitz.

Anything compressed to it's most minimum size will create a black hole.
our  earth also can be a black hole but it's size will be equal to the size of a coin.

how stars turn into black hole?

But their is no known method to turn it into a black hole.Black hole mostly form due to collapsing/dying star but our sun is not that powerful and big enough to turn into a black hole by it's own energy it needs stars 6 to 7 times larger or heavier than sun to create a black hole by this own energy

Stars gets energy through it's core where hydrogen is fused into helium by nuclear fusion which happens due to mass of star itself.
in this process there will be a stage when all hydrogen will get fused.Now,it will fuse other element  and at last iron will be left ow star cannot fuse the iron so star get collapsed in it's own core and forms either a neutron star or a Black hole.

credit for image: https://www.schoolsobservatory.org/learn/astro/stars/cycle

so, how do black hole die click here.