Data on the methods by which people are murdered in the United States can be obtained by looking at the Federal Bureau of Investigation's Uniform Crime Report (UCR). This link gives information on murders committed in the United States from 2006 to 2010.
According to the data in the link above, the vast majority of Americans who are the victims of homicide are killed by firearms. During the time period 2006-2010, roughly two-thirds of homicide victims each year were killed by people using firearms. Of these homicides, the majority were committed using handguns. Therefore, the most common method by which people are murdered in the United States is by firearms.