There are three primary seasons in Thailand rainy (May-Oct), Cool (Nov-Feb), Hot (March-May). Rainy season is the least predictable of the three; however it just rains for few hours in the afternoon or night leaving the unpaved roads muddy. The best time to visit Thailand is during cool season however the day time temperature might reach 30 °C during the day at times. If you plan to visit during hot season when the temperature reached 35 °C just go and hit the beach.
As a tourist one should be aware about Thai laws regarding passport, the age of consent and smoking in public. One should also be aware about pickpocketing and bag-snatching and con-artists.