Upon arrival, the university's international office will guide you to what you should do, step by step. Even though I arrived 3 weeks late, the welcoming festivities for international students were still going on.
If you are in Porto, you will realize that these festivities never end. The Erasmus Student Network (ESN Porto) will always keep you busy with their never-ending activities, trips and events. So, if you are going on an exchange program, no matter which country or city you choose, always check for the closest ESN section in charge; they will be of great assistance in helping you to know around.
As for the classes, they were mostly in Portuguese. However, in order to make it up for international students, the professors will always make sure to provide you with the suitable references in English. Most of the courses are also evaluated based on a single assessment at the end of the semester, which can be done again as recourse exams.