I want to write a story about a girl who is paralyzed from the waist down. Should I have her disabled in the beginning, or include the development of her disability in the story?

It depends on how much you want to explore her disability story. If it's just a fact of life to her, then you should write her as having been disabled for years because then she'll be more used to it. Becoming/discovering that you're disabled means adjusting to the fact that your life won't be the way you expected, that you have limitations that your peers don't, and that you have internalized ableism. It can be a very rough journey. Often it takes discovering the disabled community to realize that life can still have hope (because there aren't many media role models). If she's already disabled, she'll have had more time to adjust; if not, it'll be very emotional and difficult for her.