What do you have to get to win to a presidential election?
In order to win a presidential election in the United States, you need to win an overall (absolute) majority in the electoral college - i.e., more than half of all electors. Since 1964, the College has consisted of 538 seats, meaning that 270 electoral votes are required for election as President. The distribution of electors to a region is determined by that region's representation in Congress (Representatives + Senators), meaning each region is entitled to a minimum of 3 electoral votes irrespective of population. The other 535 votes are held by the 50 states - in 48 states, the candidate who wins the most votes wins all of the State's electoral votes (winner-takes-all). Maine (5 votes) and Nebraska (4 votes) are the only exceptions. The state-wide winner receives 2 votes (representing the Senators of each State), plus 1 vote for each congressional district.
There is a popular misconception that a candidate requires at least 50% of the popular vote to win a Presidential election. This is not true - it is simply a common occurrence due to the strong two-party system in the US, meaning that because third party candidates fare so poorly at a federal level, the winning major party candidate will probably have more than half the vote.
In reality, it is entirely possible and occasional the case that a candidate will win more than 269 electoral votes - thus becoming President - without winning more than half the popular vote. This system is a consequence of first-past-the-post voting, where a candidate only needs the most votes to win a given district. Furthermore, because the distribution of electoral votes is unequal and not always proportional due to population changes, it is possible for one candidate to win enough States by a thin margin to give them a majority in the electoral college, whilst his or her opponent might win all of the other states. In 2012 for example, a Republican candidate could finish first in the 11 most well-represented States by a very thin margin and be elected President, whilst his Democratic opponent could finish first in the other 39, winning the nation-wide popular vote in the process. In that scenario, the second most popular candidate is elected President even if his Democratic opponent won the other 39 states, 5 districts and D.C. by landslides, which would give him more than half the popular vote nation-wide.