The true purpose of the Second Amendment to the U.S. Constitution is to preserve the concept of the citizen militia, an armed citizenry willing and able to defend its rights against enemies foreign and domestic.
Citizen militia have a long history. For most of human history and pre-history, countries had no standing armies, but instead relied on all able-bodied males to take arms when necessary to defend the community. Rather than being obsoleted by modern technology and industrial civilization, militia can now easily include anyone who wishes to help, including women and the elderly.
With a rise of gun control legislation in the 1990s, a grass-roots movement arose to reclaim the tradition of the citizen militia. Some of the modern militia movement is rooted in libertarianism; some in conservatism; a few in fringe groups such as survivalists and racists. The media stereotype of a militia, however, only serves to keep people from understanding why it's a good idea to have as many checks as possible on the power of government.
Personal note: Although I support the idea of a primarily citizen-based national defense, I am not a member of any militia, nor have I ever contacted any of them, nor do I have any interest in contacting any of them. It's just not my style. So don't ask me how to get in touch with one, because I have no clue. If you really want to find one, do an Internet search, and if that doesn't help you, ask around at local gun stores or gun rights organizations.