Some places will ask you for loads of paperwork, so make sure you have copies of your passport and EHIC and passport photos with you just in case. On the other hand, they might not want anything from you. What is certain, however, is that whether you've given them everything or not, things still won't be sorted out straight away!
2. Try to take non-Erasmus classes.
The best way to meet Spanish people is to take classes with them, so try to avoid Erasmus-only classes, and be a little skeptical about 'Libre Elección' courses as these can be Erasmus-heavy. There's nothing wrong with hanging out with Erasmus students, it's just that many of them will want to practice their English on you!
3. 'Where's the computers?'
Of course I don't know about every uni in Spain, but in Deusto there were blackboards and projection screens as standard. No smart-boards and no comfy chairs! But it does prove how much clutter there is in British unis!
If you have to take 30ECTS, you'll have quite a lot of hours compared to your home uni. I only did 26 credits and technically I should have had 21 hours a week, but...
5. 'No class tomorrow!'
Sometimes, your tutors will fancy a day off. It'd be rude to let them down, so trade in your 'I'm paying for you to teach me' for 'we'll catch up mañana'! And if it's not down to your tutors, there'll be a saint's day coming up, and where there's saint's days there's a chance of a 'puente' - this means that if a bank holiday falls on a Tuesday, you'll probably get the Monday off too!
6. Noticeboards are for notices...
Are they? You'd think so. You might spend a fair few hours reading the noticeboards in the first few days to try and get a timetable together and things. But, just occasionally, a notice that should be there, won't be. Just take deep breaths and remember that everything happens mañana. Everything.
7. Expert Englishmen!
You may well be expected to know everything about the history/language/culture of Britain, just because you happen to be British. You can either just look blank when the tutors ask for your personal opinion on the monarchy, or you can make up random facts and try to flog them as the truth. A thousand points to anyone who manages to convince a Spaniard that eating jelly babies is compulsory whilst within a one-mile radius of Buckingham Palace as they were originally invented to pay homage to the Royal Family!
8. Keep your language on track.
It's a great idea to take note of all the new vocabulary you hear during the day, come home and look up the definitions to make sure you understand and remember new words. Collins have a great selection of eBook dictionaries which means you can look up everything on your computer or Kindle without needing internet access, and use it easily in lectures too.
9. Disfruta mucho!
Take advantage of the weekends and saints days and travel about, but above all, enjoy yourself!