Before going abroad, you should contact your home country bank for information about the extra costs they may charge you for using a debit or credit card in another EU-country. Normally this should be free, but you never know...
A credit card is always useful abroad, especially for emergencies. Visa and Mastercard are commonly used in Spain. American Express is not very extended.
It can also be a good idea to take some foreign currency with you when you first arrive.
On the other hand, the most popular card international students use is the pre-paid Currency Card, a card that you can preload with funds and is used in the exact same way as a normal bank card.
If you are not from the European Union, the best option is to open a new account in Spain. In order to get a phone, a Spanish (or European) bank account is necessary.
If you want to open a new bank account in Spain, The most popular banks in Spain are:
Documents needed for opening a bank account
Attention: In some cases, documents that aren't in Spanish may need to be officially translated. Besides this, some banks also require that your documents have an Apostille stamp
How to pay in Spain?