Find your voter registration number by visiting the website USA.gov, and from within that site search further by using its search box. The search provides links to most states' individual election commissions. You can also visit the U.S. Election Assistance Commission for the information.
Alternatively, call the toll-free number of the Elections Division of your state and inquire about your voter registration number. Another way is to visit the County Voter Registrar's office, request to look at its voters' lists and look for your name and voter registration number. The registration number can be found on the voter registration certificate issued to you upon registering.