Lots of choices. For cheap flights you might want to try:
http://asia.com. When you arrive, and if you want to do some sightseeing, Bangkok might be enjoyable. A few days should be enough.
The Islands: Koh Samaui in the Bay of Thailand - a pretty big Island that is getting very built up. Most of the diving is done around Koh Tao. A nice place but if Diving is your thing, I'd pass.
Koh Tao: A small laid backed Island with lots of backpackers. For the most part not very fancy. The diving is not as good as on the Andaman sea side but very enjoyable. The main area around Mae Haad and Sairee beach are very crowded but Chalok Baan kow is quieter and there are several good dive ops there. I like diving there and if you have a month, you can dive there as well as in the Andaman Sea.
Phuket: Patoong is crowded but Kata Beach and Karon are more reasonable. Several people on this forum work out of Phuket and can hook you up with accomodations, day boats and Live aboards. On The Spot(Jamie) at
divetheworld.com, Ringo at
phuket-scubs-club.com, and StevenL. A liveaboard to the Similans is a must, including Richilieu Rock. Hin Daeng and Hin Muang in the South are also top spots for a live aboard.
Koh Phi Phi is a favorite with some good diving, but can be very crowded.
If you would like it on the more quiet side, Koh Lanta. As a rule of thumb, the further South you go the quieter it get.
You have a month so you'll be able to do quite alot but don't try to do it all. Thailand is a fabulous Country with lots to do and great diving. I have not been very specific but hopefully provided you with a brief overall picture of Thailand and the Diving. If you have any questions, PM me.
Richard