Full answer
Voter registration in the US is handled at the state level, so requirements vary. The fastest starting point is vote.gov, the federal portal that routes you to your specific state's registration system with the correct forms and deadlines.
To register you'll need your state-issued driver's license or ID number (or the last four digits of your Social Security number), current address, and date of birth. Most states process online registrations in minutes and send a confirmation.
Registration deadlines are critical — most states require registration 15–30 days before an election. A handful of states offer same-day registration at the polls, but confirming your state's rule in advance prevents disenfranchisement.
After registering, verify your registration is active at your state's election website a few weeks before the election. Address changes require updating your registration — voting at the wrong precinct can result in a provisional ballot. This is general information, not legal advice.