In Miktex 2.9 you can choose New File > File From Template > Basic Templates > Letter.
Edit to make this clearer:
I don't know what you are using to write your Latex documents. I use a free tool called MiKTeX version 2.9 which offers some built-in templates, although you can use document classes for which there are no templates. As I mentioned it can be downloaded for free. You could search for "miktex download".
When I go to my start menu and look for Miktex 2.9, I choose Texworks. That will open an editor. Using the file menu you choose each of these choices in turn: File > New File from template > Basic Templates > Letter. This will open a file with some Latex code in it already, a template for a letter. You can change the code that is there if you want. Today I had to remove the part that makes a mailing label for the recipient because it showed at the end of my letter which I intend to post online as a cover letter. I don't need a mailing label.
I don't know about other Latex editors but you might try opening a latex document with:
\documentclass[letterpaper,12pt]{letter}
\begin{document}
\begin{letter}{recipient name and address}
if you are using a different editor that does not have templates.