He writes aphoristically, so you can dip in and out. The more you read of any of his books, the more you get a sense of his themes, and his themes are more important than his details, I think. The Genealogy of Morals is often recommended as a good place to start. The Birth of Tragedy is good because it's short and stark. If you just want a sense of his overall philosophy, you can go with an encyclopedia article, such as one of these:

https://www.britannica.com/biography/Friedrich-Nietzsche/Nietzsches-mature-philosophy

https://iep.utm.edu/nietzsch/

https://plato.stanford.edu/entries/nietzsche/