You can learn Spanish grammar and basic pronunciation online or from books, but comprehension is probably better done person to person for most. Knowing what to say is one thing, understanding the replies another! If you only want to learn enough Spanish for the Camino, you can probably get away with online or an app, but if you want to hold conversations with native speakers, a tutor or group is likely the way to go. I would recommend Language Transfer, a 'free' online resource to help people around the world speak other languages via audio lessons. The Spanish lessons can be found at
https://www.languagetransfer.org/free-courses-1#complete-spanish