Try to google learning thai (i can't post links) and click on the first site that pops up.
Also check out books by Benjawan Poomsan Becker, her dictionaries and textbooks are very good.
Then again it depends why you want to learn lah. You want to sian char bor, dun need learn until so serious. If you are really serious, you need a good Thai grammar textbook, and learn the following:
Spoken:
1) tones (there are five tones in thai, as compared to four in mandarin)
2) the various sounds (bp sound, dt sound, ue sound) and long vs short vowels
3) basic vocab - actually you go Kinokuniya in Taka got this big section on learning many foreign languages, just pick up one or two. but i recommend Benjawan Poomsan Becker if you are really serious - just that you have to learn to read IPA (international phonetic alphabet) to use her stuff properly - it really isn't that hard.
Written: (if sian char bor this is really not necessary)
1) alphabet
2) grammar