Learning a language is based on studies of structure (grammar) and meaning (semantics). The former includes the composition of words, their syntax, and sounds. The latter includes denotations and connotations. All the material that follows is based on second language acquisition, defined as a language consciously acquired or used by its speaker after neurological puberty (this is closely linked to Critical Period Hypothesis).
Learning a second language can only be realistically achieved with stages . Whereas the acquisition of a first language is a natural process, learning a second language is a conscious one .
Unless one is only intending to learn the written form, first is sound recognition and expression. This is one section, along with basic numbers, that should be rote-learned.
See also the presentation The Worst of English.
In late 2015 I started using Duolingo and have since completed the skill trees for Esperanto (March 2016), French (May 2016), German (July 2016), Spanish (November 2016), anglais (English from French) (March 2017), Italian (March 2018), Englische (English from German) (June 2018), allemand (German from French) (November 2018), Portuguese (December 2018) and Französisch (French from German) (December 2018). In March 2017 I started the Liber Lingo project (which translates as "Free Speech" in Esperanto), which I have done very little on.
 Rod Ellis, The Study of Second Language Acquisition, Oxford University Press, 1994
 Stephen D Krashen, Second Language Acquisition and Second Language Learning, Prentice-Hall, 1981