Jonas decides that he will write a letter explaining that he disagrees with the principal's decision because he believes that uniforms take away from students' individuality. What is his best next step? determine a topic

brainstorm different ideas

create an outline

choose an audience

Since the problem of using school uniforms is already defined in the question, the step that should be followed from it would be creating an outline. An outline is essential for the organisation of ideas that he will write the letter in order for him to clearly convey his opinion to the principal.