When sitting, it is considered polite to keep your feet flat on the floor or to tuck them underneath you. If you must cross your legs, do so with the foot of the lower leg resting on the thigh of the upper leg. Avoid pointing your feet towards anyone.
Additionally, it is considered rude to point your feet at religious objects or images of the Buddha. When entering a temple or other sacred place, it is customary to remove your shoes and walk barefoot.
Understanding and respecting these cultural norms will help to ensure that you don't inadvertently offend anyone during your visit to Thailand.