Mail in the United states is handled by USPS, the United States Postal Office. To forward your mail or change your address fill out the Mover's Guide on USPS's website.

For any letter addressed within the United States, the USPS requires two pieces of information on the envelope.
Address of the recipient: Placed on the front (non opening) side of the envelope in the center. Generally, the name of the addressee should be included above the address itself. A ZIP code (area code) will facilitate delivery.
Postage indication: All parcels must include an indication that postage has been paid. In most cases, this is a stamp, though metered labels are also common. Proof of payment is usually placed in the upper-right corner.
A third, and optional (but strongly suggested) addition is a return address. This is the address that the recipient may respond to, and, if necessary, the letter can be returned to if delivery fails. It is usually placed in the upper-left corner or occasionally on the back (though the latter is standard in some countries). Undeliverable mails that cannot be readily returned, including those without return addresses, are treated as dead mails at a Mail Recovery Center in Atlanta, Georgia or Saint Paul, Minnesota.

The formatting of the address is as follows:
• Line 1: Name of recipient
• Line 2: Street address or P.O. Box
• Line 3: City State (ISO 3166-2:US code or APO/FPO code) and ZIP+4 code

  Example:
  Mr Ian Porter
  112 Beacon Street
  Boston, MA 02119-2343

To find a post office near you, go to USPS's location map of New York City.

Domestic first-class mail costs 42¢ for envelopes (27¢ for post cards) and upwards, depending on the weight and dimensions of the letter and the class. The website offers a handy postage calculator. For a complete lists of prices and rates go to http://postcalc.usps.gov/.