For small amounts, I'd avoid using ATM's in Thailand, there's a hefty fixed fee (used to be 180THB for using foreign cards, maybe more at the airport).

If you end up wanting to exchange cash (to THB or back), there's several exchange offices down at the ARL station in the airport.

At checkouts in supermarkets etc. it's easy to pay with cards, but people are mostly using cash money in Thailand.

As for taxi's, the normal way is to simply wave down any passing taxi with the free sign on, or catch it after it dropped of the previous passenger.

I find the taxi's in Bangkok easier to deal with and more honest than those in my own city. Cash only I'm afraid.