Question

From where hydrogen came into sun

Answer 1

If you start with a mass of hydrogen gas and bring it together under its own gravity, it will eventually contract once it radiates enough heat away. Bring a few million (or more) Earth masses' worth of hydrogen together, and your molecular cloud will eventually contract so severely that you'll begin to form stars inside. When you pass the critical threshold of about 8% our Sun's mass, you'll ignite nuclear fusion, and form the seeds of a new star. While it's true that stars convert hydrogen into helium, that's neither the greatest number of reactions nor the cause of the greatest energy release from stars. It really is nuclear fusion that powers the stars, but not the fusion of hydrogen into helium.