Daniel, I would say that the starting point for any of these is really the same as the starting point for learning to write fiction - of whatever you want to write, read a lot of it and write a lot of it.
For instance, I built my minor career in film criticism by watching films and writing about them. I started off with a little blog, and when I look at the reviews I wrote early on I cringe at them, because I knew very little about my subject (Indian film) and very little about the craft of writing film reviews. But over the years I watched and watched and read and read and read. I read reviews by my peers and friends, and reviews by great critics like Pauline Kael, Anthony Lane, Roger Ebert, Baradwaj Rangan, Jai Arjun Singh, Raja Sen, and many others both great and not-so-great. And I got better at it, a lot better. Eventually I built a following for my own reviews and I landed a semi-regular column in an Indian news magazine, where I wrote critical essays about the movies - essays I am damned proud of, to be honest.
But the process wasn't any different from the process of learning to write fiction. Pick something that interests you, and write and read and write and read.