Please see my past answer:
Problem in typing in Hindi and english[
^].
From your comment, I can guess that you may want to provide a "virtual" (on-screen) keyboard for your Web pages. I can help there, too, but you never mentioned about that, this is only my guess.
Devanāgarī is so popular, it is supported on nearly all modern systems by default (except perhaps some separate characters; for example, the rupee sign was standardized relatively recently, so it was not included in many fonts, so Indian use some special font just to include it). I never can understand, why people from India keep asking such question, as if they almost never used their own languages on computers. Learn those simple things, after all!
[EDIT]
If you want to proceed with a virtual keyboard, you should have a keyboard with a English-Devanāgarī switch on it. This is how it is usually done.
—SA