This Side of Paradise is a bildungsroman—that is, a coming-of-age narrative. In this case, the coming of age in question is that of Amory Blaine, and the story revolves around his tempestuous personality and erratic inner monologue as he goes from boyhood to school and then to university while scrambling to figure out who exactly he should try to become in adulthood.
The novel details Amory's constant—almost frantic—efforts to become someone other than who he currently is, whatever the cost. This is made clear at the very beginning of the novel:
His chief disadvantage lay in athletics, but as soon as he discovered that it was the touchstone of power and popularity at school, he began to make furious, persistent efforts to excel in the winter sports, and with his ankles aching and bending in spite of his efforts, he skating valiantly around the Lorelei rink every afternoon, wondering how soon he would be able to carry hockey stick without getting it inexplicably tangled in his skates.
Cursed with this self-awareness, Amory reads the room, determines his self-worth against his peers, and acts accordingly. This is the template by which This Side of Paradise begins to document Amory's coming of age.