Google uses a hyperlink based algorithm (known as ‘PageRank’) to calculate the popularity and authority of a page, and while Google is far more sophisticated today, this is still a fundamental signal in ranking. SEO can therefore also include activity to help improve the number and quality of ‘inbound links’ to a website, from other websites. This activity has historically been known as ‘link building’, but is really just marketing a brand with an emphasis online, through content or digital PR for example.
All sites have a home or "root" page, which is usually the most frequented page on the site and the starting place of navigation for many visitors. Unless your site has only a handful of pages, you should think about how visitors will go from a general page (your root page) to a page containing more specific content. Do you have enough pages around a specific topic area that it would make sense to create a page describing these related pages (for example, root page -> related topic listing -> specific topic)? Do you have hundreds of different products that need to be classified under multiple category and subcategory pages?
After that, you need to make a choice about how to construct an online presence that helps you achieve that goal. Maybe you need to set up an e-commerce site. If you’re interested in publishing content to drive awareness and subscribers, look into setting up a blog. A simple website or landing page with a lead capture form can help you start developing your brand and generating traffic. A basic analytics platform (like Google Analytics, which is free) can help you start to measure how you are tracking towards your initial goal.