You mean iPhone, right? I don't think iPods have SIM cards. If you do mean iPhone you need to have it unlocked prior to changing the SIM card. If you're an AT&T customer from US you can call 6-1-1 to get them to do that, if you qualify (you have to have owned your phone for 2 years). You can also get it unlocked by third-party folks. Once that's done you should be able to go to a cell store in Sevilla (Orange, MoviStar, Vodaphone, etc.) and purchase a prepaid SIM card from them. Theoretically they should be able to install it for you.
I brought my iPhone on a couple of caminos, including this year on the Camino del Norte and I've used it in various different modes -- no cell, just WiFi, cell, etc. If you're just making calls you can consider an international plan from AT&T and it might be easier and less hassle than unlocking and re-SIMming your phone. There's also a SMS plan for not much money. Where they get you is on the International Data Plan, which is $200 for 800 mb. The way around this is just to use free WiFi at bar/cafes along the way (it's pretty plentiful) and turn off your data roaming.
Hope this helps. Buen camino