Question

what should be done to decompose water into hydrogen and oxygen gas

Answer 1

U can electrolyse it.
That way, acidulated water will break into Hydrogen and Oxygen.
If u want to try it practically, take ur phone's battery and drop it in a mug of water. The bubbles which come out will be what u want. Take a burning splinter to identify them. If it goes pop, its hydrogen. If it burns more, its oxygen.