a) Was there a specific  founder of the religion (Hinduism)?

Hinduism cannot be described as an organized religion.  It is not founded by any individual.  Hinduism is God centered and therefore one can call Hinduism as founded by God, because the answer to the question 'Who is behind the eternal principles and who makes them work?' will have to be 'Cosmic power, Divine power, God'

b) How did Hinduism start and when did it begin?

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 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).

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 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.

c) What is the most important part of the religion?

The affirmative attitude of Hinduism toward life has been emphasized by its recognition of four legitimate and basic desires:

1. Dharma or righteousness

2. Artha or wealth

3. Kama or sense pleasure

4. Moksha or freedom through communion with God or the infinite