If you use Windows Vista, Sinhala is already supported.
If you use Windows XP (Service Pack 2), Install http://siyabas.lk/files/SinhalaTamilKit_XP.exe
If you use Service Pack 3, Install http://www.locallanguages.lk/files/SinhalaTamilKit_XP_SP3.exe
If you use Linux distro such as Debain/Ubuntu, please go to http://sinhala.sourceforge.net for more information. Fedora 7 and upwards already support Sinhala.
If you Mac OS, Install the Malithi Web Font made for Mac from http://web.nickshanks.com/typography/