To write a summary of any work of fiction, start by identifying the author, title of the work, and main characters. Then, explain the most important parts of the story, which include the conflict, the climax, and the resolution. The conflict is the problem in the story; the climax is the moment that the conflict reaches its moment of greatest intensity, and the story begins leading to the resolution; the resolution is the way in which the story's problem is solved.
In Beverly Cleary's Dear Mr. Henshaw, the central conflict is that Leigh is unhappy about his parents' divorce. More importantly, he feels lonely as a consequence of the divorce because he is home alone more often and because he feels cut off from his father. He is especially upset since his father breaks his promises to phone, which makes Leigh feel forgotten and unloved. A minor conflict concerns the fact that Leigh feels bullied at his new school since someone keeps stealing from his lunches.
The story reaches its climax when Leigh feels so hurt and abandoned by his father that he wants to take his feelings out on someone. He decides to try to take revenge on the lunch thief by grabbing any random lunch bag and attempting to kick it down the school hallway, but he is luckily stopped by Mr. Fridley, who offers him some advice that helps lead towards the story's resolution:
So you've got problems. Well, so has everyone else, if you take the trouble to notice. ... Turning into a mean-eyed lunch-kicker won't help anything. ... You gotta think positively. ("Tuesday, February 6")
After receiving Mr. Fridley's advice, Leigh begins to think positively and comes up with the idea to use a lunchbox instead of a bag and rig an alarm for it. While rigging the successful alarm doesn't help him catch the lunch thief, it does help him in ways he couldn't have anticipated. Everyone at school is so impressed with his alarm that it makes him "feel like some sort of hero," because everyone at school is having trouble with the lunch thief, not just him. Seeing that everyone is having the same problem makes him feel less lonely and isolated, which helps him make friends. His making friends is a major part of the story's resolution.
The final part of the resolution is that, because he is thinking positively, he comes to see that his dad hasn't truly forgotten him and will always love him. He also forgives his father because he comes to understand just how lonely and difficult his father's job as a truck driver is.