Check with the customs office in your own country. Visit the vet. Tell your vet where you are going - certain regions may require vaccinations for specific diseases like distemper or Lyme disease. Ask your vet whether you might need to use tranquilizers. Most vets recommend against them, but in certain cases they might be appropriate.
Individual airlines have their own policies and regulations.
Get your pet used to the crate or pet carrier well before the trip. Leave it in open in the house with a blanket and some toys inside. Shut your pet inside it periodically for short periods, while you stay with it and give it attention, so the animal doesn't think it's being punished. Take your pet in the carrier on short trips in the car to assess its reaction and help it get used to the experience. Purchase a special vacation tag to add to your animal's collar which has your name and the name, address and phone number of your lodging establishment at your destination.