On the envelope, you may write "xxx收" (literally "to be received by xxx") or "xxx亲启" (literally "to be personally opened by xxx" - especially if letter is confidential).
At the top of the letter, the Chinese simply address the recipient by title/relationship, etc. Some examples would include: 尊敬的客户阁下 ("respectable & honorable customer" - writing for a company), 张博士 ("Dr. Zhang" - someone you're not especially close to), or 亲爱的大文 ("dear Dawen" - a friend). People also write greetings such as "见信如晤" (literally "seeing this letter is like meeting (in person)"). Due to influences from Japan, some people write "xxx 敬启" (literally "please open") at the top of a letter (as the Chinese equivalent of Japanese haikei 拝啓).
In the letter, you may address the recipient by 阁下, 台端 or 敬启者 if you want to be polite.
Finally, at the bottom of the letter, people write 此致 (some people write 此致 敬礼, but this looks a bit too pedantic IMO), 祝好, 顺颂时祺, or 某某顿首 (where 某某 is your name, most formal).
This should be enough for an informal and/or reasonably formal Chinese letter.