By the end of Chapter 4 of Rick Riordan's The Lightning Thief, first book of the "Percy Jackson and the Olympians" series, Percy has just successfully battled the Minotaur. Though his mother has disappeared in a flash of light, Percy has also successfully rescued Grover by dragging him into the woods to hide. In Chapter 5, Percy wakes up after having battled the Minotaur and doesn't know where he is. He is soon told that he is at Camp Half-Blood; that the Greek gods of mythology truly exist; that he himself is a demigod, or half-blood; and that he has to remain at Camp Half-Blood or else face more monsters. All of these revelations plus his new surroundings present Percy with plenty of new challenges.
One of his challenges is accepting Greek mythology as true. After hearing Annabeth talk about recently visiting the gods on Mount Olympus by riding the special elevator of the Empire State Building to the 600th floor and describing the quarrel between the gods she had witnessed, Percy feels very overwhelmed. As he explains to Luke back in Cabin 11, "I don't belong here ... I don't even believe in gods."
A second challenge he must face is bullies, just like the bullies he has had to face at all of his new school. This time the bully is named Clarisse, and she's the daughter of Ares, the god of war. As a prank, Clarisse drags Percy into the girls' bathroom. But when she does, Percy feels "a tug in the pit of [his] stomach," and as he has this feeling, water shoots out of the toilets and showers, combating Clarisse and her friends with water, driving them out of the bathroom.