First things first:
a) do you already speak, or have a basic knowledge of, Bengali?
or (b), are you planning to learn Bengali and want to write from the start?
If (a), then you need to learn the basic Devanagari layout on a ‘qwerty’ keyboard. Bengali is one of the languages that use an adapted form of the Sanskrit or Prakrit alphabet: others include Hindi, Punjabi, Nepali, Gujarati, etc. Once you have learned the basic layout, you can apply to any of these languages.
বাংলা ”bangla” is the result if you type the following on the ‘qwerty’ keyboard: b-e-x-n-e. Obviously, this bears no resemblance to the Bengali, but that is how the Devanagari letters are mapped to the ASCII codes.
An interesting site is the following also: http://www.omniglot.com/ (I think!) contact me and I can give you more details.
Check posting on Gujarati above.