Should I tell my parents that I am no longer a Christian but am now a Wiccan?

They will find out eventually and it would be better to come from you than someone else through the gvine. Be honest with them upfront and be prepared to take on the fireworks. The problem I see is that you say you are "no longer a Christian," but I would have to argue that if you turn to Wicca, you never were a Christian in the first place. Just because you are raised in a Christian home does not automatically make you a Christian.