Stars are born from large clouds of gas and dust called nebulae. Clumps of gas and dust start to stick together under the power of gravity. Eventually they have enough mass, and are dense enough and hot enough that nuclear fusion starts and the star begins to burn (this is what makes them so bright). Some of the dust and gas that is left over from the nebulae, orbits around the star and it's this stuff that groups together to form the planets. Stars which don't have enough mass and aren't dense or hot enough, become brown dwarves. Jupiter doesn't have enough mass for nuclear fusion to start and so can't become a star. It would need to be about 60 times more mass for that to happen.

I hope that helps to answer your question.

Hazel