Windows Movie Maker, as Dei linked, is an acceptable place to start, most of the time. That's how I got my start, and all the real moneymaking videos on my youtube channel (read: my first 6 Space Engineers tutorials) and the first 8 or so Halforums Academy videos all were made in Windows Movie Maker.
That said, if you really get into making videos, you'll start to want to do things that WMM doesn't really do well, such as zoom/pan/overlay with precision direction or have better fonts or use special effects such as blur on certain defined areas of the screen, etc, you'll have to look into stepping up to Adobe Premiere, which (since Adobe has moved to subscription pricing) will run you $20/mo (or $50/mo if you want all the Adobe CC products, like Photoshop, After Effects which is Photoshop for videos, Dreamweaver, Illustrator, etc).
Or maybe you know a handsome, rakish fellow who knows where such things routinely fall off trucks.
If Adobe isn't expensive enough for you and you like feeling helplessly lost and confused, you could also try Sony Vegas, which is probably by far the most powerful video editing software on the market but is only slightly less complicated, arcane and intricate as flying the space shuttle, starting at $400 and going up from there.
