I started studying Thai two months before I was due to fly out. I had a book, some tapes and a Thai friend who was willing to help me practice with him once a week. I practiced very hard to make sure I was getting the tones correct. By the time I got to Thailand I could only speak about 30 words. But I spoke them well.
My vocabulary doubled in the first two weeks there. I carried a phrase book everywhere and was constantly looking up the words for things. I guess I had the tones pretty good because people seemed to think I knew a lot more Thai than I did. Often they would launch into a long and excited monologue. I would smile and listen politely, seeing if I could pick out any stray words that I actually recognized. Then I would say the three words that have become burned into my brain: mai kao jai (I don't understand).
After 6 months in Thailand, I had learned a vocabulary of over 400 words and could recognize most the common characters of Thai script. My ability to understand Thai always remained weak compared to my ability to speak it. But I had become a master of such things as getting directions, reading menus, negotiating hotel rooms and haggling purchases. I had also memorized the words to several Thai pop songs, without ever actually learning what the hell they're singing about.
Overall, I really can't recommend learning Thai if you're not planning to spend at least a few months there. If you're at all serious about learning Thai I suggest you learn to read it as soon as possible. The various methods for writing Thai in roman characters are garbage.
I've bought or borrowed a lot of books on learning Thai over the last year, but Benjawan Poomsans works are the ones I used the most. Her "Thai for Beginners" is the only book I would recommend to someone trying to get started with Thai (get the tapes too).