How do you call South Korea from the US?

The telephone country code for South Korea is +82. From the US, dial 011-82, followed by the Korean number, omitting the leading 0.

(The plus sign means "insert your international access prefix here." From a GSM mobile phone, you can enter the number in full international format, starting with the plus sign. The most common prefix is 00, but North America (USA, Canada, etc.) uses 011, Japan uses 010, Australia uses 0011, and many other countries use different prefixes.)