Your mom said NO. And that means exactly that. You need to have a calm and mature discussion with her - and that means no whining or crying, and allowing her to speak her mind. Hamsters are nice little pets, but their cages are expensive, and they need daily maintenance (removing the soiled litter with pee and poo) and a good weekly cleaning. Arm yourself with the facts, and discuss them with your mom. Also factored in there is just who is going to pay for all of this? Go online and get cage prices - suitable ones, not the tiny things sold as hamster cages. And in the end, if she still says no, ask her what pets would be acceptable to her. There are other nice little pets in the world other than rodents.