This is a multi-layered question, that we have tried to answer in various articles. Please click on the corresponding links to learn more about the different topics. To recap:
Passport & Visa: Tourists visiting the Galapagos Islands must have a valid passport and, in most cases, can obtain a visa upon arrival in Ecuador. Click here to read more.
Luggage Check: When leaving Ecuador's main land, tourists' luggage will be checked for restricted items by the Tourist Control and Certification (TCC) to ensure the preservation of the islands' unique environment and wildlife. This check costs 20USD and is done at the airport in Quito or Guayaquil, before checking in for your flight to Galapagos.
Travel Insurance: A travel insurance that covers emergency evacuation and medical expenses. is highly recommended, and even obligatory on board of some ships.
Vaccinations: At the following link you can find out more about vaccinations for a Galápagos trip.