Stars initially form from clouds of dust and gas. The force of gravity makes the gas and dust spiral in together to form a protostar.
Gravitational energy is converted into heat energy, so the temperature rises. When the temperature gets high enough, hydrogen nuclei undergo nuclear fusion to form helium nuclei and give out massive amounts of heat and light. A star is born. Smaller masses of gas and dust may also pull together to make planets that orbit the star.
The star immediately enters a long stable period, where the heat created by the nuclear fusion provides an outward pressure to balance the force of gravity pulling everything inwards. The star maintains its energy output for millions of years due to the massive amounts of hydrogen it consumes. In this stable period it's called a main sequence star and it lasts several billion years. (The sun is in the middle of this stable period).
Eventually the hydrogen begins to run out. Heavier elements such iron are made by nuclear fusion of helium. The star then swells into a red giant, if it's a small star, or a red super giant if it's a big star. It becomes red because the surface cools.
A small-to-medium-sized star like the sun then becomes unstable and ejects its outer layer of dust and gas as a planetary nebula.
This leaves behind a hot, dense solid core - a white dwarf, which just cools down to a black dwarf and eventually disappears.
Big stars, however, start to glow brightly again as they undergo more fusion and expand and contract several times, forming elements as heavy as iron in various nuclear reactions. Eventually they explode in a supernova, forming elements heavier than iron and ejecting them into the universe to form new planets and stars.
The exploding supernova throws the outer layers of dust and gas into space, leaving a very dense core called a neutron star. If the star is big enough this will become a black hole.