The sun uses hydrogen as fuel mostly through a series of nuclear reactions called the proton-proton chain, leaving behind helium ash.
Just the bare facts ....
In the proton-proton chain, 4 protons are converted to a helium nucleus, energy (carried by gamma rays) and some very light particles (neutrinos). The helium nucleus has 7-tenths of one percent less mass than the 4 protons and other stuff added together. The missing mass amounts to about 25 million electron volts (MeV) (4E-10 joules) of energy.
100 trillion trillion trillion helium nuclei must be created every second to produce the 400 trillion trillion joules of energy we observe leaving the sun in a second.