How is a black hole created?

General black holes are created by dying stars. If a star has a mass that is about 20 times greater than the mass of the Sun, it may become a black hole when it dies.

Generally, the gravity of the star is constantly pulling in and the pressure pulling out. So, there is a conflict between gravity pulling in and pressure pushing out. Nuclear reactions in the core of the star produce enough energy and pressure to push outward. For almost the entire life of the star, gravity and pressure balance each other exactly, and so the star is stable. However, when a star runs out of nuclear fuel, gravity  is more and the material in the core is compressed even further. The more massive the core of the star, the greater the force of gravity that compresses the material, collapsing it under its own weight.

A star is called a "white dwarf", when the star the cools and dies peacefully because the nuclear fuel  is exhausted and there are no more nuclear reactions to fight gravity, the repulsive forces among electrons within the star eventually create enough pressure to halt further gravitational collapse. 

If the core remaining after the supernova is very massive (more than 2.5 times the mass of the Sun), no known repulsive force inside a star can push back hard enough to prevent gravity from completely collapsing the core into a black hole.

From the point of view of the collapsing star, the core compacts into a mathematical point with virtually zero volume, where it is said to have infinite density. This is called a singularity.

Where this happens, it would require a velocity greater than the speed of light to escape the object's gravity. Since no object can reach a speed faster than light, no matter or radiation can escape. Anything, including light, that passes within the boundary of the black hole -- called the "event horizon" -- is trapped forever.

Bibliography-1).http://hubblesite.org/reference_desk/faq/answer.php.id=56&cat=exotic