What should I do if I want to become a Hindu, but my mother will not allow it?

Try to talk to her and convince her why you want to become a Hindu. Unlike Abrahamic religions which are belief based, Indian religions are practice based. so it's the practices which will make you a good Hindu. If your mother doesn't allow it, then keep your previous religion officially and start practicing whatever you learn from Hinduism. No one, irrespective of religion, would have a problem with you if you decide to become a vegetarian or want to meditate daily, Start practicing the omnipresence of the divine by treating humans, animals, plants and Mother Earth with respect.