'Sawasdee krap' (men) or 'sawasdee ka' (women) is hello. Use it the first time you meet someone, not repeatedly.
Other useful Thai: 'khob khun krap/ka' (thanks), 'mai phet' (not spicy), 'aroy' (delicious), 'tao rai?' (how much?).
Learning even basic Thai earns goodwill. Tones matter — listen and try to copy.