I'm moving back to spain and I want to take my fiat barchetta with me but I heard that you have to change the licience plates over. How would I do this and how much does it cost? Is there anything else I should know?

Usually there is a grace period to accomplish this. During this grace period you can drive on your US license plates (as long as the car is also properly insured) until the paperwork clears. Once the registration/importation paperwork is accomplished they will issue you a registration number. Most European countries do not supply license plates they supply the specs such as size and color and you either make them yourself or have them made by any shop that offers that service.