\documentclass[a4paper]{letter}
\signature{L. A. Tex, Software Advisor}
\begin{document}
\begin{letter}{William Gates\\Win Street 311\\Redmond, Wash.}
\opening{Dear Mr. Gates,}
I'd like to show you, how simple it can be to use \LaTeX: Just write
down your text, don't care for spaces or lines, and \LaTeX will set it
for you nicely.
Getting new paragraphs is very easy: Just insert a blank line. Of
course there are more advanced methods which allow you to control
everything, if you don't like the indentation or the spacings.
This should be enough for the very beginning.
\closing{Best wishes}
\end{letter}
\end{document}