Here's a link that may help:http://www.usps.com/postalhistory/s...andPostcards
To speed things up, this is taken right off the US Postal Service website, which should answer your question:
Alexander M. Greig's City Despatch Post, a private New York City carrier, issued the first adhesive stamps in the United States on February 1, 1842. The Post Office Department bought Greig's business and continued use of adhesive stamps to prepay postage.
After U.S. postage rates were standardized in 1845, New York City Postmaster Robert H. Morris, among others, provided special stamps or markings to indicate prepayment of postage. These now are known as Postmasters' Provisionals.
On March 3, 1847, Congress authorized United States postage stamps. The first general issue postage stamps went on sale in New York City, July 1, 1847. One, priced at five cents, depicted Benjamin Franklin. The other, a ten-cent stamp, pictured George Washington. Clerks used scissors to cut the stamps from pregummed, nonperforated sheets. Only Franklin and Washington appeared on stamps until 1856, when a five-cent stamp honoring Thomas Jefferson was issued. A two-cent Andrew Jackson stamp was added in 1863. George Washington has appeared on more U.S. postage stamps than any other person.
Until government-issued stamps became obligatory January 1, 1856, other payment methods remained legal.