Take some coaching lessons first, so you learn the basic techniques and how to serve.
Just a warning but starting at such a late age might mean you find things like serving extremely difficult to learn, so you may get a lot worse at first before you get better. I would also point out that playing in tournaments will most likely be a massive jump up in level compared to school, so you'll probably suffer some heavy defeats so brace yourself for that!
Sorry if it sounds like I'm pissing on your parade, but I just remember myself having great ambitions when I was younger only to find out how insanely high the level was once you step up to county level, but maybe you really do have talent in spades and can do well.
Good luck either way