Find an ATM with a physical sticker on the console that reads "Global ATM" -- in the city these will usually be at bank locations. Incheon or Gimpo will have these, for obvious reasons. (Random subway or convenience store ATMs without a Global ATM sticker, though they will often have English and Foreign Card selections in the menu tree, will work maybe 20% of the time at best.) Use whichever card you're most comfortable with fees-wise -- a Visa will likely work best -- and if one card doesn't work, try another, then switch ATMs. I called back to my bank a few times while in Seoul earlier this year, and found some transaction attempts simply never reached them from even some Global ATMs.

Korea is not on the EMV (chip-and-PIN/chip-and-signature) train yet. You'll be swiping for everything.

You'll need cash to buy and load your T-Money cards. Honestly, when I looked at it before our recent first trip, I decided peace of mind upon arrival was worth sacrificing the cost of a cheap dinner out, and bought a few hundred thousand KRW from a local bank so I didn't have to worry.

Have a great trip!