A ninja is a member of a Japanese society. Ninjas are highly trained in stealth and martial arts, and were typically hired for covert operations and assassinations.
A novice is someone who is only just beginning out in a new job or a new area of learning. The term novice is most often used to mean someone who has just become part of a religious order, like a monk or nun.
A nun is a woman who has taken vows to devote her life to a religious cause. Nuns usually live in a special place like a monastery, away from mainstream society. Nuns wear a special uniform, so you can tell who they are.