There are many excellent resources on the WWW for learning Thai. The best I have found for written Thai is the Northern Illinois University SEAsite, where you will find a variety of textbooks covering everything from Janet-and-John to university level, converted to multimedia formats with sound files, pop-up translations and Java self-tests. The site also covers the surrounding countries and their language and culture.
Paknam Web hosts a wide range of websites, many of which began as school projects, including Learning Thai Language the Easy Way.
For a one-volume Thai-authored primer for English speakers, see The Fundamentals of the Thai Language, which is presented both as HTML and as a Windows Help file. The only drawbacks of this book are some typograhical errors, and the "phonetic" transcription of the Thai. The phonetics have been improved in a later edition which is unfortunately not yet available on-line.
The Courage Software Interactive Thai CD-ROM is a good product, designed in conjunction with the Australian Defence Force School of Languages, and well worth the cost.
For setting up a Windows PC to use Thai keyboard and fonts, (essential if you intend to read the soc.culture.thai newsgroup) see the ZZZthai pages.
The most readily available textbook for British readers is Teach Yourself Thai, which is pretty good apart from its confusing phonetic transcription, which assumes you speak Received British English. It is definitely worth buying the accompanying audio cassettes.
A better alternative, if you can find them, are the new books by Benjawan Poomsan Becker: Thai for Beginners, Thai for Intermediate Learners, Thai for Advanced Readers from Paiboon Publishing. Each comes with its own CDs or cassettes. Unlike some of the Thai-published alternatives, these books have been produced to high editorial standards by native speakers of both Thai and English.
However, there seem to be no readily-available English-language texts which go much beyond the elementary level.
Most of the Thai you will meet speak a little English, but unless they are highly educated, they will probably use Thai rhythms and pronunciation rules, which can make it difficult to understand. If in doubt, ask them to write it down. There is also a tendency to translate colloquial Thai phrases into literal English, which can sometimes make social platitudes sound deceptively peremptory. For example, the word "Khun" translates to "Mr/Mrs", "Sir/Madam" , "excuse me" or "you". If a passer-by says "You!", although it sounds like a challenge, they are probably just trying to practice their English. Similarly there is a meaningless phrase used like the English "how do you do", which is often translated literally as "where you go?". Again, it isn't a challenge, just a greeting - and the answer is "I go around".
The Thai alphabet is very different from the Roman one, which causes very variable transliterations. Street names are displayed in Thai and Roman, as are destination signs on the roads, but spellings may vary. For example there are many streets in Bangkok with names referring to the King. The name may begin "Ratcha-", but at the other end of the same street will be the more recognisable "Raja-".