How to learn to produce documentaries?
While there are tons of tutorials on the internet, which keep you learning your whole life (which isn't a bad thing) I would propose one option that I followed myself.

Lynda.com has good quality videos about all aspects of documentary filmmaking - technology, conducting interviews, storytelling, editing, ... 

When I knew I had to take the next step in documentary filmmaking, I took one month off and watched these videos from morning to night. After one month I knew enough of all major topics and could cancel my membership. All in all I paid only US$ 25 for 30 days of intensive learning.

After that learning period you just need two things: Practice and errors. Because you'll only learn when something goes wrong. That's why I suggest that you only buy simple gear that you can upgrade after you found out what can go wrong.

Here's a small gear list: a video-capable DSLR, be it Canon, Nikon, Sony, Panasonic, ... Then a tripod and a microphone solution (e.g. a handheld audio recorder like the Zoom H2n). Light is not so important, if you can find bright locations.

When it comes to editing software, there is no single best choice. Some people prefer Adobe Premiere, because together with the other Adobe programs they have thousands of possibilities, but with a heavy price tag. Others find Final Cut Pro best to realize their creative visions. Edius is known to be a professional editor that runs fast even on slower computers. The free Resolve would be an option, if you want to do editing now, but later want to become a pro in color grading. There is also Sony Vegas and the free Lightworks,  ... What you will choose depends wholly on which direction you want to develop. The editing process is nearly the same with all of them.