there are a few decent editing programs out there, but the most common one is Final Cut Pro (FCP). A cheaper version of the same thing is Final Cut Express (FCE). But both of these programs only work with a Mac. (Speaking of which, iMovie comes absolutely free of charge with the Mac, and is a very handy program for beginners.)
if you have a PC, there are a myriad of options, the most popular of which are Avid and Adobe Premiere. unless you are wanting to be a professional editor, Avid is probably too expensive for you and requires too much of a learning curve. Adobe Premiere is easy to learn and widely used but not so much by documentary filmmakers.
all in all, if you can afford it, get the cheapest new Mac you can get, and start editing with iMovie. after a month or so of practice, then shell out the few hundred bucks for FCE. when you have a project that's actually fit for broadcast, upgrade to FCP.