Hi Anna-Rose ~
Credit cards are not essential, unless you're staying in hotels and making lots of advance bookings for rooms. Most people bring a credit card for these situations and/or for emergencies. A debit card is essential so that you don't have to carry lots of cash. Most villages do not have an ATM, but mid-sized towns on up do have ATMs. Plan ahead so you always have cash for 3-4 days, as there are some stretches of the camino (notably Astorga to Molinaseca) where you walk for that long without encountering a town large enough to have an ATM. I've never had a problem in Spain with my US debit cards. Many people recommend calling your bank to be certain they know you're heading to Spain and will have multiple debit transactions there. You'd hate for them to think your card was stolen and cancel it while you're there.
Most pilgrims stay in the albergues. You'll want a guidebook or albergue listing so you can make a plan each day. Most English speakers find the John Brierley guidebook Camino Pilgrim Guides: Camino Santiago de Compostela, Frances, Finisterre and Portugues most helpful. Make certain to look for the current (8th) edition. You'll find that albergues range in price from "donativo" (which doesn't mean "free" but means "whatever you can afford") to 10-15€. Some of the best albergues are the very smallest ones in which the hospitaleros put together an evening meal and have some sort of program or ritual. Examples are Granon, San Bol, Tosantos, San Nicolas, etc. Buen camino!