Thailand has never been colonized. One drawback about this is that most people speak only Thai language. However, learning English is mandatory in most schools. People in big cities can understand and speak English. Tourists will not have problem in Bangkok and other cities. For the remote areas, the tourists may have to invest a bit more attempt to communicate with local people.
Central or Standard Thai (or you can call it Bangkokian language) is understood by most people because it is taught in all schools and is the language used in all official media in Thailand. Apart from Central Thai is also spoken Southern Thai, Northern Thai and Esan (Northeastern Thai) which is very similar to the Laotian language. The local dialects can be very different from Central Thai with different pronounciations and even different words for the same thing.
The Thai language can seem a bit complicated at first. This is mainly because it is a tonal language, which means that what sounds like the same word for a westerner can be pronounced with five different tones, each tone giving the word a different meaning. The five tones are: Medium, Low, High, Falling and Rising tone (in addition to this some Southern dialects have an additional sixth tone, the high rising tone). If you are staying longer in Thailand and want to learn the language then take a language course in one of the plentyful language schools. One of the best (but unfortunately also one of the oldest) learning systems for Thai is AUA. The AUA language material consists of reading, writing and conversation materials as well as audio material. AUA language schools are also situated in the bigger cities of Thailand.
If you manage to pick up some words do not hesitate because of fear for pronouncing the words with wrong tones since Thais normally understand the meaning out of context. If pronounced wrongly there is a big chance that people will understand you and politely correct you by repeating the word with the proper pronunciation. However much or little Thai language you know, do not be a show-off because Thais hate that. If you just politely try to communicate with people in Thai language they will admire your efforts no matter how good it actually sounds and probably encourage you to learn some more.
As mentioned English is widely understood in the big cities and you might find that people will be very happy to talk to you if not just to practice their English. Some people pronounce the English words after Thai pronunciation rules which means that it sometimes sounds quite funny for a Westerner (or Farang as it is called in Thai). One example is Siam Square which should be pronounced "Saiaam Saquare" after Thai pronunciation rules. Another funny example is that the Scandinavian male name Sven should be pronounced Saven (when pronounced it sounds like Seven). This is not only Sven's that find it funny but also Thai people find it entertaining.
The Thai script is a syllabic alphabet based on the Brahmi script adapted to write the Thai or Siamese language. Its invention is attributed to King Ramkhamhaeng, who reigned from 1275 to 1317. It is also possible that the Khmer alphabet might have had an influence on the Thai alphabet. The oldest Thai inscription dates from 1283.
The Thai script has some very complex rules on how letters are pronounced. Often multiple letters represent the same sound. This is partially due to the fact that many sounds found in Indian languages did not exist in the Thai language, so letters representing similar sounds in Indian languages came to represent the same sound in Thai, and also due to phonological changes in the Thai language in the past hundreds of years that have merged different sounds into the a single sound.
On the other hand, the sounds of many Thai letters differ depending on whether the letter occurs at the start or the end of a syllable. There are only six consonants that can end a syllable, namely /p/, /t/, /k/, /m/, /n/, and /ng/, but nearly all letters can occur at the end of a syllable (but pronounced as one of these six consonants).