What does the main character want?

In the story, the main character is twelve-year-old Martha Boyle. From the text, we can conclude that what Martha most wants in life is to be a famous writer.
The events in Martha's life during an eventful summer lead her to see life from a different and more mature perspective. She begins to learn about death, life, and love—all topics Jimmy (her summer crush) is also exploring. She also learns about the meaning of trust, family, and loyalty. All of these unique experiences contribute to Martha's growth as a budding writer.
The book addresses death at the very beginning, when Martha receives a brief, awkward visit from Olive Barstow's mother. Olive, Martha's former classmate, died in an accident. Olive's mother gives Martha a page from her daughter's diary, in which Martha is mentioned in positive terms. On the page, Olive also expresses her desire to see the Atlantic Ocean and become a writer. Thus, the author uses this unique introduction to segue into Martha's story as she spends the summer at her grandmother's home on the Atlantic Ocean.
Martha's experiences during the summer, although painful at times, serve as a conduit to personal growth. She becomes acquainted with the intricacies of love (whether sexual, familial, or platonic in nature), the pain of betrayal, and the mysteries of death. The title of Jimmy's film The World Is Not What You Think It Is is significant, as it characterizes much of what Martha discovers during the summer. Martha learns about the ephemeral nature of life when she nearly drowns while trying to hide from Jimmy's line of vision. While staying with her grandmother, Godbee, Martha finds that she must come to terms with the fact that Godbee may die soon.
Despite the negative experiences, Martha also learns about loyalty and kindness. Tate, Jimmy's younger brother, tries to make up for Jimmy's cruel behavior through a thoughtful gesture. Martha's experiences add to her emotional maturity and contribute to her growth as a budding writer.