If you are not a U.S. citizen, become a U.S. citizen.
If you are under 18, wait until you are 17 yrs. & 11 months.
Register to vote in your county of residence. You can probably find the necessary form at you county's tax office, your county clerk's office, a post office, and often at offices of your state's Division of Motor Vehicles (DMV). You can probably also download it from your county's web site (http://co.<YourCounty>.<Your2LetterState>.us). Be aware that most states have a minimum time period between when you register and when you can vote.
Check the web page of the Elections Division of your county for the dates of upcoming elections.
A few weeks before the election, download a sample ballot for you precinct if you do not automatically receive it in the mail, so you know who or what is up for vote. It's not always people; sometimes it's proposed amendments to the state constitution, or anything else for which state law requires voter approval.
Familiarize yourself with the candidates and the questions on the ballot so you can make educated choices. Shortly before each election, local chapters of the League of Women Voters put out Voters' Guides which are an Excellent source for just enough information on which to base your choices. As an alternative, you may choose to go with the endorsements of a person or organization that thinks like you do.
If you have to be out of town on Election Day and your state does not have in-person early voting, get an absentee ballot from your county and send in the completed ballot before the deadline.
On Election Day, go to the polling location designated for your address. Follow the directions of the election workers, and cast your votes. You don't have to vote for every position or question on the ballot, in case you can't confidently make an informed choice for every race due to insufficient knowledge or whatever.
