Apostasy is leaving ones Religion. A former believer can leave and convert to another religion or can leave to become an Atheist, Agnostic, Freethinker. Adherents of the former religion tend to see apostasy as betrayal and in a few Islamic nations apostacy is punishable by death.
Youngsters from Christian families who become atheists
Should you tell your parents? If you're under 16 you should almost certainly wait till you'r older. Don't face the firestorm that may follow till you're able to handle it. Please read How stereotyping atheists as immoral can do harm, my experience to see what can happen.