Why doesn't Holden tell the readers very much about himself or his prior history at the start of the novel?
In the opening chapter, the style of narration employed by Salinger is deceptive. The narrator, who the reader later discovers is Holden, often directs his comments to "You" who the reader may think is himself or herself. In the opening chapter, Holden is in a sanitarium so you can presume he is speaking to a psychologist about what happened to him once he left Pencey Prep. This story begins at its end and then Holden takes us on the journey that lead him to the sanitarium.
Holden's disconnect with adults is a theme explored throughout the book. He seems to rebel against the loss of innocence and childhood. Holden doesn't talk about himself directly because it's too painful for him. Instead, you have to analyze what Holden says about others to see what he is trying to reveal about himself.
The story is told as a flashback. Holden is in a mental institution at the beginning of the book, he then begins to tell the reader the story of how he ended up in the mental institution. We already know how the book ends. How Holden ends up and through reading the novel, we get to understand why.
The author allows the reader to experience Holden's feelings and draw conclusions about why he feels the way he does.