Most stars are made of hydrogen.and Helium.

THere are huge clouds of gas and dust in the Galaxies.They are called Nebula..Due to some reason mass gets accumulated at center and this mass attracts more gases..Then it become a p-roto star.When mass become very huge due to pressure and temperature at about 15 million degree K. at center nuclear fusion stars changing hydrogen into helium..4 hydrogen atoms fuse into 1 helium atoms and remaining 0.7% mass converted into energy. Picture credit science how the stuff works.com.