I'm assuming that, by a 'letter', you mean a complete document (of the type which you post), rather than a single character (such as a, b or c).
Documents are never 'in' any program on your computer. They're simply in a specific location on your computer, from where they can be accessed by any suitable program. So you don't delete them from within a program. Just go to where the document is stored (e.g. 'My Documents'), right-click on the relevant file and select 'Delete'
WordPad is an extremely basic word processing program. I'm amazed that anyone actually uses it. If you've not got Microsoft Word, download and install the free OpenOffice suite of programs, and use Writer instead:
http://www.openoffice.org/
Chris