Does a spanish citizen need visa to usa?
Unless you have any criminal convitions you will not need a vise pre-approved before hand. As you are part of the E.U you will just need to fill in a green form on the plane and give it in to customs in the U.S. They will ask you of your intentions and adivise you that you may not stay any longer than 90 days. You will be given part of the form back which you must surrender when you leave the U.S. Make sure you have a passport valid for at least 3 months from the date you fly in and that it is an electronic one.
