The ETIAS visa waiver system is a travel authorization that was launched by the EU. All eligible non-EU citizens, including Americans, are required to apply for an ETIAS authorization to visit any of the 26 member states of the Schengen Area.
To apply for ETIAS, you will need to complete an online form with your biographic information and answer a questionnaire. You will also need a valid travel document or a machine readable passport. At the time of the application, passports need to be valid for at least 6 months and cover the entirety of the trip. If your passport has already expired or is about to and does not fulfill the requirements mentioned before, then you will need to apply for a new passport before applying for an ETIAS visa waiver. If you apply for ETIAS with your old passport and then travel with a new one, you will not be able to enter the Schengen Area.