Pick up a martial Art, learn to stand up for yourself.
Dont pick a bs sport like Taekwondo, do something like BJJ or Boxing.
You should also start to consider losing some weight if you're fat, or gaining weight if you're skinny, vice versa. This is why I got picked on initially - for being fat.
Remember to never hit the first punch. A detention is a far better option than having to get bullied the rest of your school life.
I wish you the best of luck mate.