The one thing I wished I would have known before going on exchange is…
Not much, because I prepared a LOT, but I wish I knew more about how the exams worked at my host school. It was hard to find information about it, and the school offered 4 different types of classes, which was confusing, but it's something that can be easily explained by a student that's been through it before.
My favorite part of going on exchange was…
Speaking Italian, hiking in the Alps, meeting other Erasmus students, travelling (especially by train), grocery shopping (unusual, but the store was nice!), and the food.
A tip I would give to students applying for exchange is…
My top recommendation is to learn, at least a little bit, of the language of your host country. Locals will always be ten times happier to help you if you just show a little effort to speak their language! In my case, most people in Turin didn't speak English well at all, so a little effort on your part goes a long way. Also, find out the visa requirements as early as possible, because you have to gather a lot of documents and physically mail them (for Italy, at least, but in general it's something you should look into early).
The most unexpected part about exchange was…
I really didn't expect it to be so easy to make friends. The absolute best thing you can and should do to make friends in your host country is to join the WhatsApp group for the local Erasmus group. There's one in almost every city, and it's essentially a group for other international students to connect, participate in events and activities, and be there to help each other. I was nervous at first, but once I met other international students, I realized that everyone else was in the same spot as me; everyone is just looking to meet new people!