Keyword Selection (or sometimes called Keyword Research) is the single most critical step in all SEO. If you don’t know which keywords will catch* internet surfers, everything else, every other step of search engine optimization is compromised.
*I think “fishing” is a good analogous activity to SEO. Keywords to SEO is like bait to fishing. If you have the wrong bait, you will not catch fish; or perhaps you will catch the wrong kind of fish.
This article has been updated here How to do keyword analysis for search engine optimization in 2018.