Firstly, although it may seem like an obvious point, it's really important to begin Erasmus with an open mindset. If I had known this before, I would have prepared my meetings well in advance before the deadlines to submit the Learning Agreement etc. Universities, especially in France, will not tell you everything, so make sure you go and see the relevant people as soon as you arrive. Check all the emails you have received from them before you depart, as they will not necessarily remind you again in September. I forgot to check one that I had received in July and they almost refused to sign my papers. I strongly advise that you get your attendance letter signed the day you arrive if you can.
Secondly, look for accommodation online before you go there. Look out for Erasmus groups in your region on Facebook or for Spanish Erasmus check erasmus.su. Another option, especially if you want to live with native speakers, is to stay at a hostel for a few nights at least a week before your placement starts and have a look around. Make sure you inspect everything as not all landlords will show you everything.
In terms of choosing courses, my suggestion is to try out as many lessons as you can for a week and see what you enjoy. That way you don't miss out on anything, and will learn as much of the Target Language as possible after having chosen something you like and feel you would understand. It is ill advised to choose another language unless it is one of the languages you already study at Durham, hereby teaching you how to switch between them. However, if you are studying in Spain, make sure to take an advanced French class in order to really target your learning in both languages and NO English classes (except translation). See if anything is related to your Target Language Research Project (TLRP).
Lastly, definitely sign up to any 'buddy' programmes. Go out alone asking for directions instead of using Google Maps so you can talk to people in the Target Language. If you have English-speaking Erasmus friends, suggest you meet other native speakers as a group through a Tandem Programme. Attend as many social and cultural events as you can to start speaking straight away!
Bonne chance à tous!