It's a huge mistake to assume one can act "just like at home" in any foreign country, especially if you want to mix with locals. Even when I was in Korea, I realised there were basic differences in my concept of "politeness" compared to theirs. (That said, Koreans are very polite. ^^) A basic rule is to watch what they do first. ;) Jura and 프레디's answers are a good place to start. And remember to bow, especially when meeting people, and also when paying money.

This site has extensive etiquette rules for Korea in all aspects of life http://www.kwintessential.co.uk/resources/global-etiquette/south-korea-country-profile.html

If you do what you do in US in Korea, It'll be acceptable, or even some people follow you as there is no western ediquate accustomed in Korea, You may feel awkward by some Korean people too much care about you as you are white & acting freely, If you want to follow Korean way, You must be very polite & generous to elder people, I do not think you need to do so, Just do what you 've been doing in US ~ Plus, If you understand Korean spoken language, plz simply ignore or do not care people gossip about you on street or public place like in a subway or in a theater =) They like gossiping , but not really nicely, that's a problem I can only briefly explain like above, as your question isn't specific... If you want to know it for each situation, ask me again