Let's look at King's Toolbox recommendations.
The tool used most often goes on top: Vocabulary. It is obviously essential. However: "One of the really bad things you can do to your writing is to dress up the vocabulary, looking for long words because you're maybe a little bit ashamed of your short ones. This is like dressing a household pet in evening clothes. The pet is embarrassed and the person who committed this act of pre-meditated cuteness should be even more embarrassed." (p.117)
Usually, the first word that comes to mind is the best one.
Vocabulary doesn't always have to come from the dictionary. You may sometimes create a word, particularly in dialogue. King quotes a great example of street language by another author on page 117. (Yes, I'm trying to entice you to read King's On Writing, A Memoir of the Craft - because you'll be a better writer for it.)
Next to Vocabulary, you'll want Grammar. King's main pet peeves are passive sentences and adverbs. He gives many examples of the difference between passive and active expressions. You want the subject of the sentence to be the active party. Adverbs should be used sparingly - if you've chosen the right verb, and adverb will weaken its impact. As for grammar overall, King was an English teacher, but he says "One either absorbs the grammatical principles of one's native language in conversation and in reading or one does not." (p. 118-119) This goes back to developing an ear for language.
As I've said before, reading is essential to develop that ear. If you grow up in a household that speaks something other than standard English, it's even more important. Online writing often strays far from correct English. Read classics, read today's good writers (including King, but you can ask a librarian or English teacher for recommendations). There are also style guides, textbooks, and classes (often at your library, high school, or community college) that offer rules, instructions, and practice.
So, Vocabulary and Grammar are the top layer of King's toolbox. Underneath you'll find Strunk and White's The Elements of Style. This is also where he discusses paragraphs at length because paragraphs are "the place where coherence begins and words stand a chance of becoming more than mere words." (p. 134)
It’s not reading until you understand the meaning of each paragraph; it’s not writing until you’ve created meaning with each paragraph.