By default, Max already has a template called "* Letter" (without the quotes) that opens automatically when you click the Letter button. Simply make the changes you want to this and go from there. If you've deleted the "* Letter" template, the Max editor will simply open an untitled document. From here, you can create a new letter or open a different template.
If you're wondering how to change the "Letter" button to open something other than the "* Letter" template, I have no idea. Probably something in the registry that needs to be changed.