Install a Thai font for your operating system. You can choose from free, public domain fonts, or you can purchase a particular one you like. See the Thai Fonts page at into-asia.com for some public domain TrueType fonts.
Choose whether you'd prefer to type with a virtual keyboard (where you use your mouse to activate the keys and "type" on your computer screen) or a real, tactile keyboard.
If you have a real keyboard, decide whether you want to use a standard English keyboard with Thai sticker overlays or switch to a Thai keyboard when typing Thai language items. Using Thai keyboard overlays allows you to type in English or Thai using your standard English keyboard, so you won't have to switch between keyboards in order to type in either language. See the Thai Keyboard Labels link at datacal.com if you would like to purchase Thai keyboard overlays for your English-language keyboard.
If you want to go with a virtual keyboard, find a Thai keyboard program that you'd like to use. Currently, several exist to choose from. Picking one largely depends on the keyboard layout you prefer and how easy you find each virtual keyboard program to use. Some examples include the Puttipan Thai Keyboard and Dr. Yes' English/Thai Keyboard Emulator.
Install Thai/English language recognition software so that you can switch between Thai and English easily when writing documents. A good (but very basic) freeware program for this function is ABC Thai. If you'd prefer a program that's more user-friendly, purchase a Thai/English word processing program for your operating system, such as GlobalWriter.
