Rumsfeld's upbringing isn't really very interesting; upper-middle class German-American family from Illinois, Boy Scouts, Princeton, ROTC, marriage at 22, kids, bit of time in the Navy. He started in politics in a pretty normal way, as a congressional aide to David Dennison of Ohio and then Robert Griffin of Michigan.
He then worked at a banking firm for a couple of years in the early 1960s, but then ran for Congress in 1962. He won that race and served four terms. He was a generally moderate Republican at this time and supported civil-rights legislation. He also co-sponsored the Freedom of Information Act, an ironic move given his later career.
But during these years, he was exposed to a vile force that has done tremendous damage to the world—the Economics Department at the University of Chicago. This transformed his views, as these ideas placed the seeds of evil in so many people over the decades and all the way to the present. How much did Milton Friedman come to love Don Rumsfeld? He later bemoaned Reagan selecting George Bush as his vice president as the greatest mistake of his presidency (how dare he use the term "voodoo economics!") and claimed that if Reagan had listened and selected Rumsfeld instead, "I believe he would have succeeded Reagan as president and the sorry Bush-Clinton period would never have occurred." Hoo boy. What a world that would have been....