How did Hinduism start and when did it begin?

Hinduism is God centred. Other religions are prophet centred.

Hinduism is based upon Eternal Principles. Eternal principles apply to all human beings everywhere. The laws of physics exist and work all the time. The healing principle will get to work immediately the moment a little cut is sustained on a finger. No one can tell when this healing principle began or when it will end. It is there existing eternally, all pervading (available everywhere), omniscient (aware all the time and therefore healing principle gets to work when injury is sustained). (These simplified examples serve to understand God’s power: omniscient, omnipresent, omnipotent).

Hinduism is based upon Eternal Principles. If a great scientist like Einstein, discovered or realized laws of physics, Hinduism would call him a great Rishi (Maharshi or seer of truth.) Such seers of truth are not confined to any one age or country. Self realized persons like Jesus Christ would be called Rishis (seers) and their teachings would be readily acceptable to those who properly understand the principles of ‘Hinduism’. From the ancient times, many great Rishis achieved self-realisation through such practices as meditation and austerities and they realised knowledge concerning Eternal Principles. Their knowledge, taught to disciples, and eventually made available in written form, is known as the Vedas (Ved = knowledge), the scriptures upon which Sanatan Dharma (Hinduism) is based. Sanatan means eternal and Dharma means religion.

The word 'Hinduism ‘ does not appear anywhere in Hindu scriptures, The proper name for Hinduism is ‘Sanatan Dharma’
Sanatan = eternal Dharma = religion.

Hinduism is God centred whereas other religions are prophet centred. For this reason the whole of mankind has to abide by (or is affected by) the eternal principles. The question of acceptance or rejection of Hinduism by any individual simply does not arise, or is irrelevant. It is illogical to talk of conversion to Hinduism. It is like saying that the laws of physics (e.g.gravity) will apply to you only if you belong to an organization or organized religion