The author of War Horse is Michael Morpurgo, a writer from the United Kingdom.
While every author develops an individual voice and ends up writing in their own unique style, Morpurgo offers a bit of insight regarding his craft on the "About" page of his official website.
According to his site, Morpurgo studied French and English when he attended university. After graduating, he became a teacher and that's when he decided he wanted to become a writer. Morpurgo lists "living in Devon, listening to Mozart, and working with children" as some of his primary inspirations for writing and says that he daydreams until his stories eventually "hatch" out (Morpurgo).
Regarding his creative process, Morpurgo says that he spends "several months" dreaming up a story in his head before he actually begins to write. He claims that this "dreamtime" is the most important part of his creative process and when he finally starts writing, he does so very quickly. He says that he usually finishes writing a book "in two or three months" and then "might take another month" revising it (Morpurgo).
That said, writing is different for everybody. It's a skill that can be learned, refined, and perfected - and there's no one "right" way to do it. It's not necessary to major in English in order to become a good writer and you don't have to be naturally "gifted" with words either. If you want to improve your writing, just practice. The more often you write, the more progression you'll see!