I want to address something that seems to be incorrect by another posting to answer this question.  In the end of the novel, Steve Harmon does not commit suicide by shooting himself.  Instead, he, since being found not guilty, has returned home and continued to work on his films, particularly ones in which he films himself. Steve has continued this work because he wants to use them to find out who he is.  Steve also questions the fact that O'Brien (his attorney) turned away from him as he reached out to hug her after being found not guilty.  He wants to know what she saw.

The book begins with Steve already in jail, arrested for being a lookout for the robbers who killed the store owner. Steve explains how he's making a movie of the the events of his trial and his past leading up to the trial. Then there are several flashbacks throughout the book that provide the reader with the background information we need to understand the circumstances of the trial. Several witnesses testify who have been given reduced sentences for placing Steve and King, the guy accused of the murder, at the scene of the crime. Steve's feelings and thoughts are also included throughout the book, showing how scared he is that he'll spend his youth in prison and how scared he is that he's the monster the prosecutor accuses him of being. At the end of the trial, Steve is found innocent, but the book doesn't end there.

Steve's last entry is in December, five months after the end of the trial. He's spent his time at home making films of himself because he wants "to know who I am. I want to know the road to panic that I took. I want to look at myself a thousand times to look for one true image." Steve is still haunted by whether he is the monster he was called in the trial. His mother is happy he's home, but he feels he's growing apart from his father since his father "is no longer sure of who I am." At the end, Steve wonders what his lawyer saw when she looked at him at the end of the trial that caused her to turn away. Then the book ends.

Here is the "overview" of Walter Dean Myer's novel about 16 year old Steve, who is on trial for murder:

"Monster is presented as a screenplay, with handwritten comments, by the main character Steve Harmon. Steve says that he is writing the screenplay to keep his sanity while being held in prison during his trial for murder. "I am so scared. My heart is beating like crazy and I am having breathing trouble," he writes. "The trouble I'm in keeps looking bigger and bigger. I'm overwhelmed by it. It's crushing me." In a story about a young man who gets in deep trouble by being greedy and wanting to look tough, the sheer terror of prison and the prospects for conviction are conveyed in blunt descriptions. Monster, one of the most horrifying novels ever written, is titanic in its sheer terror.

In the end, Steve commits suicide by shooting himself.