Internal links do two things: they let Google discover pages, and they tell it what those pages are about through the anchor text. Both are fully under your control, which is rare.
Crawlable links
Google follows <a href="...">. It does not follow buttons with click handlers, elements with onclick, or links whose href is generated only after interaction. If a link matters, it must be a real anchor with a real URL.
Link text
Descriptive anchors help. “Compare our SEO packages” tells Google and the reader what to expect; “click here” tells neither anything.
Depth
Pages many clicks from the homepage get crawled less often. Important pages should be reachable within a few clicks.
Alien Road recommends: audit which of your own pages receive the fewest internal links, then fix that before buying a single external link. On most sites we assess, the money pages are under-linked from within while the blog links generously to itself, free authority sitting unused.