The ID number
This identification number for foreigners is a personal, exclusive and unique sequence of characters, issued by the Immigration Office. As you might use it often, we advise you to memorize this number. It will also be necessary for any type of transaction, to open a bank account or sign a contract with a telephone company.
How to proceed?
To obtain your NIE, it's very simple: you just have to go to the nearest police station at the Foreign (European in your case) department with your identity card and a copy of it, a passport photo with a white background, the original copy of your lease and your documents concerning your Erasmus. Without these papers, you can not apply for a NIE.
Once you have completed the application, you will have to wait several days or even several weeks, before getting your Spanish ID number at the police station.