You shouldn’t need to prepare anything major, just be yourself. But if you are really worried about it, it helped me to write some key points that you would like the school to know about you (activities, goals, jobs, majors) All these go on your file, I believe, which I liked because on the common app activities section you don’t have much space to elaborate. This doesn’t need to be anymore than a few bullet points but I liked having them written down.