Nietzsche totally blows my mind each time.
Ecce Homo is a good place to start, actually it’s one of his last books. It’s a small book and it’s made of small sections, which makes it easy to read on and on/off basis (this does not happen all that often when it comes down to intellectual literature so it’s worth noting).
It’s not a auto-biography but he sort of goes through his works and what each book meant to him and so forth. So it’s an easy read, and an excellent intro to his oeuvre, also shedding light on several key concepts of his.
After reading this book you can either leave it at that if it’s not your thing and the book will probably have done a better intro job than any of the ‘companion’ books around (these are probably longer and more difficult to read anyway!). Or you can read more, if you’re keen, and in this case this book will really help you know which of his books you’re interested in.