\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.