The unusual chain of events that lifted Ford to the Oval Office began in 1972 when operatives connected to President Richard Nixon's (1913-1994) re-election campaign broke into the Democratic National Committee headquarters at the Watergate Hotel in Washington, D.C. A number of high-ranking Nixon administration officials knew about the break-in, and the president himself took part in efforts to cover up the illegal activities that became known as the Watergate scandal.
As the scandal came to light, Vice President Spiro T. Agnew (1918-1996) resigned from office in October 1973 over unrelated charges of accepting bribes and evading taxes. Nixon used his power under the 25th Amendment to the U.S. Constitution to appoint Ford as his new vice president. The well-liked and respected Ford was easily confirmed by Congress and took office on December 6, 1973.
Source: History Channel-http://www.history.com/topics/gerald-r-ford