how stars are formed?

From the gravitational collapse of a cloud of gas. Look up 'solar nebula', 'protostar', or 'stellar evolution' for more details.

All stars are formed from nebulae (the plural of nebula). Nebula is a term for a cloud of gas, and stars form from gas. Stars more massive than ~ 6 solar masses are expected to supernova, stars less massive than this (like our Sun, of course) become white dwarfs. After a supernova, there may be nothing left, or there could be a remnant: either a neutron star or a black hole. If the remnant is more massive than around 3 solar masses it will probably end up as a black hole. Stars are smallest when they are burning hydrogen into helium, which is what stars do during most of their lifetimes. Stars in this stage are sometimes called dwarfs. There are also two other kinds of "dwarfs": white dwarfs are burned-out stars mentioned above (the Learning Centre has more info on these), and brown dwarfs are stars which never accumulated enough mass to start burning hydrogen.

To make a star.

Gather a huge amount of hydrogen gas together in an area of space that is both cold enough for molecules to exist and in the near vicinity of a star about to go supernova but not too near because the full force of the nova would just make a mess.

The gravity of one little molecule will attract a slightly less massive molecule to itself. That will give the joined pair even more gravity because it has more mass. This will go on and on for a while, the "ball" of hydrogen getting bigger and more massive with much more gravity pulling it all to center, which get's VERY HOT with all the hydrogen atoms juggling around bumping into each other. And, if the timing is right, there will be sufficient mass and gravity that when the nearby supernova happens to give it that final "push" with it's shock wave that the center of the mass will reach around 10 to 15 million degrees Kelvin. That's the "magic" temperature when hydrogen atoms are so squished and excited and energetic that they just have to fuse and ....A star is born. Ta Da!