In the 2000 election, George Bush won the state of Florida by 537 votes. Your one
vote would have been .19% of the total needed to help Al Gore. So our decision of whether or not to vote really does matter. These instances are rare, but they do happen.
More likely, your one vote will be the deciding factor for others' votes. It takes a combined effort to make a whole nation vote. Your one vote might not directly affect the election, but it can help convince others of their civic duty(responsibility of them as American citizens) to vote.