There is one good expression: "To change the world - start with yourself!"
This principle is valid for search engine optimization as well. This applies to internal optimization of website.
If you want to optimize your site with SEO (in German: suchmaschinen optimierung), then you should start with this internal optimization. It will be very useful for the initial promotion of your site in search engines. This process is very time consuming, but when done right, it gives a good result.
In addition, internal seo is not one-time event. This means that you must keep track of the content published on your site constantly (if there are other people who post content) to sharpen published article, promoted by key words (phrases), and also to replenish their list, if necessary, but within reasonable limits. All information should be sorted into groups of related subjects with one or more keywords of your site. And most importantly - it is unique content that you post.
This is only part of the internal optimizations, since the inner seo happens all the time, as you add headings, content, etc.
In addition, there is a foreign concept optimization - website promotion through backlinks.