Get a job, get good grades, otherwise be a model child
I was not able to start getting pets of my own unless I saved up and bought them myself. Find a way to earn money (if you're too young for a job, try negotiating extra chores for extra $$) and save it up until you can afford the snake, cage, and food. That is usually a great way to show responsibility and maturity deserving of a pet snake.
-Jen