it's nice to hear you wish to become a witch, but you need to know it's different than what you see in movies. you cannot change into other things, you can't fly on broomsticks, and so on. do you have any idea what path you wish to follow? Witches are always studying and learning, some cast spells, but many don't, or only cast when it's needed. do you wish to simply cast spells, or want to learn about the spiritual aspects as well?
start with daily meditation, it'll help you with your magic but health wise as well. as you get better with meditation you can do energy work, shielding, and balancing your chakra's. these will help you as well.
if you're interested in Wicca, try reading Wicca for life by Raymond Buckland and/or Wicca: A Guide for the Solitary Practitioner by Scott Cunningham. there's tons of books and sites to get you started, try looking through the articles on the site for some help as well.