Will good things happen to a person if they do good? Does karma exist? So in other words: If one share with the world everything they have without expecting good to happen to them in return, will great things happen for them anyways?

Surely not every good action will be recognized or rewarded by others, but most people would benefit from living in a world where many people perform good acts and so contributing to the existence of such a world might be a good goal to strive toward even if you are moved neither by a desire to improve your own soul (as Plato might have it) nor by the prospect of doing good out of a sense of duty and respect for morality and rationality (as Kant might have it).