If you meet someone in the USA who you don't know, it's usually polite to shake hands. Some people may go in for the hug, but generally you can get away with a quick handshake. As I've said in getting around the USA it does depend a lot on where you're visiting, as the USA is a big place.
Speaking of big places, make the most of the space and keep an arm's length of personal space between yourself and others.
The USA is a melting pot of people and as a result the topic can be quite sensitive - depending on who you're talking to. If you must reference race, Black or African-American, Asian, Latino or Hispanic, Native American or American Indian, and White or Caucasian are acceptable terms. But I'd just avoid the subject all together and concentrate on having fun.
Some Americans may abuse me for saying this, while others may readily agree, but thanks to the media output and education in America, citizens can often be unfamiliar with the rest of the world. Don't be offended if they ask things like 'Is London in England, or England in London?" as they did me, or whether you know Prince William - just go with it.
Like most places nowadays smoking is seen as a dirty habit and is frowned upon - don't even think about doing it indoors.
There are Native American reservations scattered throughout the country. If you find yourself visiting one on your gap year, just be aware that the site is most likely acred to the tribe, and certain places may be off-limits to outsiders. Just respect the land and the traditions and you'll get along fine.