It varies by state (I assume you're in the USA, since you said "president"). Many states now have registration at the Motor Vehice office, where you get a driver's license. Often there wil be registration drives when the deadline approaches, with a table at the supermarket or shopping mall (usually sponsored by whichever political party is in the majority in that neighborhood). Be sure to find out if you must register in the same county you live in. As Blind_Of_Lies said, be sure to check with the county clerk or election office to learn the local rules, and to find where and when to register--as well where, when and how you can vote. Then follow through, get informed about the candidates and issues, and be sure to vote!