Your thesis statement should include an opinion or assertion about the topic (symbolism in A Farewell to Arms)and provide the main sub points that will be the basis for your body paragraphs in your essay.
The first step is to decide what symbols you want to write about. Here are some possible choices: rain, Catherine's hair, Catherine's riding crop, officers' stars, Henry's beard.
Now, you could write a thesis like: Hemingway uses ___, ___, and _ ___ as symbols in A Farewell to Arms. BUT, that is not a very high level thesis statement. The problem with this thesis is that it fails to make an assertion. If you want to go higher level (and probably get a better grade) then you should connect Hemingway's use of symbols to some deeper meaning or theme in the novel.
Look at the possible themes from the novel: "patriotism, individualism, war, and identity" (from eNotes/themes page).
When we pull it all together, then we should have a thesis like this:
Ernest Hemingway reveals insight and perspective on the theme of __________(pick a theme) through his symbolic use of_____, ______, ______ (choose three symbols that you can best discuss) in A Farewell to Arms.
Example: Ernest Hemingway reveals insight and perspective on the tragedy of war through his symbolic use of officers' stars, Catherine's riding crop, and rain in A Farewell to Arms.