Thai is the country's national language. It has five tones, which makes it confusing to most foreigners. Thankfully, Thais tend to be patient with people trying to speak their language, and will often guess what you are trying to say, even if it's badly mispronounced. In polite conversation, a male speaker will use the word "krup" to end a sentence or to acknowledge what someone has said. Female speakers use "ka." It's easy to speak a few words, such as "sawahdee krup" or "sawahdee ka" (good day) and "khop khun krup" or "khop khun ka" (thank you).
With the exception of taxi drivers, Thais working with travelers in the resort and tourist areas of Thailand generally speak sufficient English to permit basic communication. If you find yourself truly unable to communicate something important to a Thai, he or she will often start grabbing people from the street at random to see if they speak English to help you out.