you can get info/forms from most major department stalls or the tourist office. basically for any shop that offers the VAT rebate, you need to spend a minimum of 2000 baht. then to get the refund at the airport the total spending has to be a min of 5000 baht.

other rules is you must enter/leave thailand by air. and i believe min 3 days in thailand.

you get 7% off with the VAT