You can change the world with one vote, but you can't vote if you're not registered. Most jurisdictions have September or October cutoff dates for registering in time for the November 2 general election. To find out the cutoff time for your area, go to http://www.anapoliticalpower.org and click on "Election 2004 Action Center" and then "Election Dates and Voter Registration Deadlines by State."
From that page you can learn more about how to register in your area by clicking on the link to "online voter registration tools," which offers links to numerous helpful Web sites.