Answer:
Black holes are made just before a star dies. The stars core radiates outward pressure to balance out the gravity pushing down on it. When the cores energy runs out gravity gets the upper hand and compresses the star. As the star gets compressed the core heats up and implodes and swallows itself. The bigger the star the more gravity there is acting upon it meaning a bigger black hole.
If the star is made of tougher stuff it becomes a white dwarf or a neutron star. Hope this helps!