For most of its lifetime, a star is a main sequence star. It is stable, with balanced forces keeping it the same size all the time. During this period:
The Sun is expected to be a main sequence star for billions of years.
In a main sequence star, hydrogen nuclei fuse together to form helium nuclei. This happens in several steps, but one way to simplify the overall change is:
Two hydrogen nuclei fuse to produce a helium nucleus and a neutron.