Immediately after you disembark your flight to Bangkok, head to the immigration lines. Make sure you have completely filled out your departure and arrival cards, which should have been handed out on your flight. If they were not handed out, there are blank ones at the immigration station. From there, simply present your passport and cards to the immigration officer. Once you have picked up any baggage at the baggage claim, you will go to customs. There are two lines here, the green line for those with nothing to declare and the red one for those with something to declare.