To be able to type in Thai, you need to have both a Thai font
installed on your computer and a piece of software that lets you type in
Thai on your keyboard. A Thai keyboard (i.e. a keyboard with both the English
letters and Thai characters on it) is not technically necessary, but it's
very useful as without one you won't where the Thai letter you're looking
for is located. If you don't have one, we have a page showing the layout
of a Thai keyboard that can be used to type in Thai, albeit probably
very slowly.
With or without a Thai
keyboard, you won't get very far if you don't have a program to switch
between typing in English and in Thai. Short of getting a copy of a Thai
edition of Microsoft Windows, probably the best and most sophisticated program
to use is ThaiMaster.
This costs 1500B to purchase but you can obtain a free 30 day trial copy
from their website. It's also readily available to buy at Panthip
Plaza or you may be able to purchase it online from DCOThai.
There is also a free option which, although it comes with no added features,
essentially works just as well. You can download it from http://thaigate.rd.nacsis.ac.jp/ftp/
thaisoft/new/HuiKey10.zip, and you should find the setup very simple.
If you have any problems, the Nacsis
Thaigate site is helpful and a good source of alternative and related
downloads. There is also another similar program called ABCThai,
which is also free to download albeit with some restrictions.