Are you traveling to or via the USA? It is then mandatory to apply for an electronic travel authorization (ESTA) before the trip. Note: even if you have a change in the US and are flying on to a destination outside the US, it is mandatory to apply for ESTA. We recommend that you apply for ESTA immediately after you have booked your trip. Due to more stringent checks, the application is not confirmed at once. Within 72 hours before departure, it is no longer possible to apply for ESTA.
Apply for ESTA via the official ESTA website.