In Western countries people say hello and shake hands. In Thailand we say sawat-dee and wai each other. Gently put your palms together in front of your chest while bending your head down toward your hands. Keep your fingers close together. Wai is pronounced like the word "why" in English.
Many of you might be more familiar with the Japanese gesture of bending when saying thank you or "Arigato Gozaimasu". Thais however, don't bend the head as much nor as often as the Japanese. Thais bend the head by lowering the neck, while the Japanese bend the whole upper body (from the waist and up).