Expanding on one of the excellent suggestions above, symbolism is frequently used by the Modernists, such as Hemingway, and this novel is replete with such a literary device. So, charting the use of such symbols as rain will later be a great aid when responding to analysis and interpretation of A Farewell to Arms.
One major benefit of keeping a reading response journal is that it draws you into the novel. It requires that you become an active reader. When I assigned reading journals, I made it an open-assignment; students were free to respond in any number of ways. I only asked that they date their entries and write in some detail.
Here are a few of the kinds of entries they would write:
They would write an entry explaining what they didn't understand after they had just read, being specific.
They would respond to a character or a character's actions.
They would write about experiences of their own or people they knew that something or someone in the novel reminded them of.
They would write about their emotions when they read certain scenes. (Henry's being wounded and rowing across the lake frequently generated emotional responses.)
They would write about themes, conflicts, and symbolism they identified.
They would take a particular passage that impressed them for some reason and write about it.
Again, I'm not sure of your assignment guidelines, but reading journals are most valuable when you write regularly as you read your way through a novel.
I feel there are several ways to progress with writing a reader response journal to any book. Before proceeding, I should suggest that if there are specific instructions that your instructor has given, then those would naturally take priority. However, if the assignment is a straight reader response, I think that the best way to go is to analyze the characters and your response to them. For example, what are your thoughts about Frederic? He is designed as the prototypical Hemingway hero, a "man's man, one who exhibits "grace under pressure" in his capacity to endure difficult times. What are your perceptions about him? How do you feel about his attitude before the war and how it changes throughout the course of the novel? How can you identify with him? If you cannot identify with him, what is it about him that prevents you from being able to do so? You can also discuss Catherine or other characters and apply the same questions to them. I think the essence of a reader response is simply that: A response to some aspect of the work. How characters are depicted, how they change, what they are like, and whether you can relate or identify with them is where I suggest students go in writing a reader response.