there are people in this world who will believe it's evil regardless of what you do or say. at some point you need to draw a line in the sand and move on. if they are willing to listen, sit down and calmly explain what you believe. get a few introduction to Wicca books [if you own it, highlight the important parts, or put stickie notes near them] or print off some facts about Wicca off credible sites, again highlighting key points. talk calmly and clearly about what you believe, the religion, and why this completes you. ask them to just read the points you highlighted. that's really all you can do.
i don't know of anyone in my family who thinks my religion is evil, but i'm very select as to who i tell, so i know of a couple people who would react negatively if they found out. you should pick and choose until you're over 18, then make up your mind how you want to live your life; loud and proud, select, or in the broom closet. i've met all three types, it's all down to personal preference. good luck to you, blessed be.