Tags are subjective to the content of your post. You want to use tags that are relevant to what you've written for that particular post or page.
For example, if I were to write a post about my experience starting a business and my content was very targeted and revolved around that topic then my tags might look something like
business, entrepreneurship, entrepreneur, starting a business, how to start a business
As Karen stated, tags are meant to be used as somewhat of a navigation piece for your users. You'll want to use the same tags for similar posts so users can click those tags and see related posts that you've previously written.
When the title of your post or page, permalink, content of the post or page, meta keywords and description are relevenat to each other Google and other SEs (Search Engines) will see this relevancy and reward you with higher rankings. On top of that, you'll want to focus on keyword density in your content. Don't stuff your content with certain keywords or phrases over and over. Use them naturally but also use those keywords or phrases in your permalink, meta keywords/descriptions, and tags.
SEO is a whole other beast in itself. There's onsite SEO and offsite SEO. Your onsite focuses on page load time, bounce rate, internal linking, title tags, meta keywords and descriptions, keyword density on your pages, having an XML sitemap to name a few and the list goes on.