You can get cash at an ATM in the airport. Global ATMs are plentiful throughout Seoul as well. But, I'd suggest using your credit card. While it's totally safe to carry around a ton of money, it's just an extra worry (to me).

Exchange your money at any bank. You'll find someone there to speak English. Also, if you give them dollars, they'll get it. Don't go to the sketch change counters in Itaewon. Even at the airport, the exchange rate is very fair and accurate. The fees are also minimal (think the exact opposite of the Travelex booths in US airports)

I've only seen one bar, Gecko's, in Itaewon that lets you use the USD. Nowhere else will take it. Plan on KRW only.

Random fun fact: There are no ATMs past security at ICN airport. You can only exchange inside the gate area.