\documentstyle[]{letter}
\signature{James Tetazoo}
\address{3 Ames Street\\Cambridge, MA\\02139}
\begin{document}
\begin{letter}{Jack Florey\\
5 East Street\\
New York City, NY\\
12345}
\opening{Dear Jack,}
body of letter here..
\closing{Sincerely,}
\end{letter}
\end{document}
\address defines what will be printed as your return
address; \\ is used to separate lines. The address
of the recipient is specified in the braces right after
\begin{letter}. If you want to have a Cc list at the bottom of your letter, this is easily done. Just put
\cc{person1\\person2\\person3}after the \closing{}; there can be as many cc'd people as you want.