Mac
In OSX Snow leopard:
in Systems Preferences => Language & Text ("International" in previous versions)=> Input Sources: scroll down and check the box for Thai ("Thai - PattaChote" is an alternative keyboard arrangement you can also use), then quit Systems Preferences.
Then to type in Thai, just click on the country flag next to the clock on the top right of the screen. This open the menu to select the active language, so you just click on Thai. From this menu, you can also click "Show Keyboard Viewer" to make a clickable on-screen keyboard with the Thai character arrangement appear.
OSX comes with 5 different Thai fonts included. Ayutthaya and Thonburi are the two that are the easiest to read.
Additionally, many more fonts for both Macs and Windows can be found at
http://www.wazu.jp/gallery/Fonts_Thai.html.
To make your physical keyboard bilingual, you can easily and cheaply purchase Thai keyboard stickers from Amazon.com and other online-retailers. Just do a search for "Thai keyboard stickers". Or if you are in Thailand, computer shops will have them.
You can practice typing with a tutorial program. I found a good free program for Mac users for learning to type Thai or any of 35 other languages here: http://mac.softpedia.com/get/Educational/aTypeTrainer4Mac.shtml
Windows
For setting up your Windows computer to type in Thai take a look at this very informative blog entry from Thai 101 (no relation to us): http://rikker.blogspot.com/2008/06/setting-up-thai-keyboard-in-windows.html
Please share any other useful resources that you've found.
