By preparing a great report for discussion on the ups and downs, risks and joys, legal aspects and animal welfare law etc etc so they can see you are aware its not all fun, that animals are people, and they are not cartoon drawings.... Impress your folks.
Tony took my answer (or I took his, depending in your prospective). Earn money, pay to get the pet as well as its ongoing care. Better yet. Wait till you're at least 18, get a job, move out, pay your own bills, and have all the pets you can afford.