My new article explains the difference between formal and informal English and shows where you can get formal and informal input.
The most important piece of advice for an English learner is to get lots of input. But not all input is the same. There are, roughly speaking, two basic types of English input: formal and informal.
Formal English is used in “serious” texts and situations — for example, in official documents, books, news reports, articles, business letters or official speeches. Informal English is used in everyday conversations and in personal letters.
I have also updated my article “Reading is easier than listening”, refining my discussion of why it is easier to understand written English than spoken English.
You left out instant messenger and texting for the informal side of written content.