Blackholes form from a massive explosions of a star. It is these forces created during this explosive event that activates a black hole. Stars have a enormous amount of mass, and once that mass hits a critical level it detonates, much like a nuclear bomb. The denotation is called a super nova and can be seen from billions of light years away. When this event occurs it creates pressure deep with in the star, and those pressures squeeze the mass that is near the center, and pushes the mass that is in the outer area of the star away into space. It is the squeezing of all the atoms, mostly of iron at this time together that force all the electrons to be hurled out into space.
Once all the atoms are stripped of electrons by the massive force that forces them together, the nucleus of the atoms are then squeezed together. Since normally atoms "size" is created by the electrons once all the electrons are gone the mass can occupy a much smaller space. One teaspoon of the stuff made is as heavy as the whole earth. At this stage we have a neutron star, where all the star is made up of is nucleus's of atoms. If the stars explosion is big enough, has enough mass, a black hole could form instead of a neutron star.
The black hole that forms can vary in size, but once formed the stuff that is made of is different than that of a neutron star. The blackhole's gravity is so immense that it squeezes the protons and neutrons together so tight that it forms a singularity, a place where space and time are also squeezed together. A singularity is a very mysterious object. We do not know what it is made of, but we do know what it is made from. How the energy and mass changes to form a singularity is very complex and poorly understood. What we do know is that once the blackhole is formed it will continue to grow if there is mass close enough to it. The gravitational pull of the blackhole will suck in anything that gets close to it. Stars are usually what feeds blackholes and the blackholes will continue to grow as long as there are stars, mass and energy to feed them.
The pressure realized to push together the proton and neutron are immense. It is in this shoving together of mass into a smaller and smaller space that the singularity starts to form. Mass and space and energy all interact with each other. Mass can be turned to energy and energy to mass. When enough mass and energy are occupying a certain volume of space/time the blackhole forms. Time behaves in strange ways not understood around the vicinity of a blackhole, and hence so does space. The formation of a blackhole is fairly common event in the universe, almost every galaxy has a blackhole in it center, including the Milky Way our galaxy.