To contact your Congressperson: http://www.house.gov, look up your representative's page.

To contact your Senator: http://www.senate.gov, look up your senator's page.

At each of the pages there should be links for constitutents to send messages by email, which are reviewed and read and tallied by the Congressperson's/Senator's staff. Originally worded ones get more attention than mass mailings, which tend to be checked over only to get a count of how many there are.