All U.S. citizens, Canadians, Australians, New Zealanders, and South Africans must have a passport with at least 2 months' validity remaining. No visa is required. The immigration officer will also want proof of your intention to return to your point of origin (usually a round-trip ticket) and visible means of support while you're in Scotland. If you're planning to fly from the United States or Canada to the United Kingdom and then on to a country that requires a visa (India, for example), you should secure that visa before you arrive in Britain.
Your valid driver's license and at least 1 year of driving experience is required to drive personal or rented cars.
Websites listed provide downloadable passport applications as well as the current fees for processing passport applications. For an up-to-date country-by-country listing of passport requirements around the world, go the "Foreign Entry Requirement" Web page of the U.S. State Department at http://travel.state.gov.