As muurta said, most places minors cannot choose their religion without their parents permission. You need to tell them, but you don't have to tell anyone else unless you want to.
Best to time it, wait for a moment when both your parents are home and relaxed, not after a stressful day. Ask them to sit down and listen to everything you have to say first. Calmly explain why you wish to become Wiccan, what Wicca is and isn't, and provide facts be it in books, printed off the internet or links for them to look into. When finished, sit and listen to them, don't cut them off or get mad [even if they are] but ask them to think about it and read the information, a week or so later, ask again, you might need to find a compromise, or wait until you are legally an adult to convert.