OK...Do you get an allowance? If so, skip to the next paragraph. If not, ask your parents about doing some extra chores for some extra cash. Don't be greedy, be fair. Give them good work for good money, save it. While saving your money, continue with the next paragraph...
Buy at least these two books: "Cornsnakes: The Comprehensive Owner's Guide" by
Bill and Kathy Love, and "Cornsnakes in Captivity" by
Don Soderberg, and start reading them. Read them cover to cover, and then again...and again. These two books will teach you everything you need to know about keeping cornsnakes as a pet. When you think you got it down...let your parents quiz you from them. Show them that you know what you're doing.
Meanwhile, you have been saving every penny, and pricing everything you will need at your local pet shop. When you have enough money to pay for
everything you need (including tax) according to those 2 books, talk to your parents in a calm and respectful manner. No begging or whining...be an adult, or your ship is sunk. If they say no, don't go out and blow the money, keep saving it. Eventually, when they realize this isn't some "passing phase" you are going through, they
might give in and allow you to buy yourself a cornsnake...