i need to make my device display in "sinhala" language
What do you mean by "display"? If it is just to read and write the script for email and websites, then all you need is a Unicode font and a keyboard layout. iOS already has the font: Sinhala Sangam MN. So it can already display Unicode Sinhala (සිංහල) text. Non-unicode Sinhala would never be supported.
If you need to have the entire user interface in Sinhala, then you have to add all the localization files which translate every menu, dialog, and warning into that language.
what about the keyboard?? is that available or do i have to create that too???
Yes, you would have to create a keyboard layout. There is, however, an app called IndicNotes which already has layouts for several other languages, and it would probably be easy to modify that (or ask the author to do so).
Have you already joined the iOS developer program?