How can a star create a black hole

It's answer can be given through the equation E=mc^2. The star which is made up of huge mass is cnverted into tiny, where the mass has gone? The mass is acually converted into huge amount of gravitional energy through the above relation.

Because when it becomes a black hole it starts to accumulate all the mass around and just keep gaining more mass and more gravitational pull and more mass

A star is a big sphere of hydrogen that is collapsing in on itself because of gravity. However, during a stars lifetime it is fusing elements together in the core (for example, hydrogen fusing into helium) which releases vast amounts of energy and pressure; pressures strong enough to balance out the inward pull of gravity. Now for the most massive stars, they will continue fusing elements in their cores until the cores themselves turn to iron. This is the end of the line because no more energy can be released from fusing iron into more elements. Once this shuts off, there is no longer the outward push of pressure and gravity wins, as it crushes the star out of existence. Now for less massive stars, there are two things that can happen. The first is that for stars that are less then 1.4 solar masses, gravity won't win completely. As gravity tries to crush these stars, the electrons of the atoms making up the star will actually create an outward pressure as gravity tries to push them together. This electron degeneracy pressure will actually balance against gravity and the star will become a white dwarf. Now for stars that are between 1.4 and 3 times the mass of our sun, gravity will force the electrons to merge with the protons of the star forming all neutrons. However, just like the electrons in a white dwarf, the neutrons will exhibit a repulsive force against gravity (because neutrons are fermions and thus can't occupy the same quantum state) which is strong enough to halt the star's collapse. This type of star is called a neutron star. Now for remnants of stars with masses that are larger than 3 times the mass of our sun, there are no known laws in physics that can intervene to stop gravity; gravity wins. Gravity will crush the star so much that it will become a singularity; a point with the mass of the star (which was at least 3 times the mass of our sun). Because so much mass is crammed into what physicists can only describe as a point, spacetime curvature becomes infinite. In physical theories, the existence of a singularity signals the edge of the applicability of the theory: in other words, there must be more to it. Quantum theories of gravity, such as string theory, may be able to avoid the appearance of singularities by effectively setting a minimum distance scale below which space time does not behave in the manner described by Einstein's equations.

Hope this helps. :)