Spain uses the euro (EUR, €) as its sole currency, which means there's no need to change money as you travel across most of mainland Europe on your gap year.
One problem with the euro is it means bureau de changes can be hard to come across unless you're in major cities like Barcelona and Madrid. You can exchange money in banks (although some might require you to have an account with them), international airports, and some big hotels.
The vast majority of shops will not accept any other currency, with the exception of within airports.
You can also use credit and debit cards incredibly widely in Spain, with even small places often accepting them. Most ATMs will let you take out money on your foreign card, though check in advance if your bank will charge you.