You cannot become a full witch at 15, most covens won't accept you until you are 18, but you can begin reading and learning about the craft. Go and join the local library. There should be plenty of books on the subject. Also talk to people online, read forums and grow in knowledge. Pay attention to the environment and the changing of the seasons. Being a witch isn't like they show it to be on TV or the movies, but it is a wonderful, caring way of life. Contact me if you've got any questions, I'll be happy to talk to you. I've been a witch since I was 15, so for 18 years now.