In order to register to vote you must be: 1) a citizen of the United States, 2) a resident of the state in which you're planning to register, and 3) at least 18 years old at the time of the election (or 30 days before for most states).
However, if you are in prison or on parole for the conviction of a felony, or if you are deemed mentally incompetent by a court of law, you are not eligible to vote.