In Rick Riordan's The Lightning Thief, first novel of the series titled "Percy Jackson and the Olympians," the rising action starts around chapters 8-10 when Chiron tells Percy to consult the Oracle to see if he should go on a quest. The Oracle does indeed prophecy that Percy will go on a quest heading west and "shall find what was stolen, and see it safely returned."
It becomes critical for Percy to venture on a quest for a couple of different reasons. First, Chiron has realized Percy is actually the son of Poseidon. According to Chiron, the three most powerful gods--Zeus, Poseidon, and Hades--vowed to never again father children with mortal women. Second, it has become known that Zeus's lightening bolt has been stolen. Plus, Zeus and Poseidon are bitter rivals; therefore, once Zeus learns Percy is Poseidon's son, Zeus is likely to accuse Percy of being the thief. But Chiron actually suspects Hades of having stolen Zeus's weapon. So, the only way to prevent a war between Zeus and Poseidon from starting is to take the lightning bolt from Hades and return it to Zeus.
Therefore, Percy, Annabeth, and Grover venture to Los Angeles, where they will be able to enter the underworld and confront Hades, a quest that leads them closer to the novel's resolution.