How to add Structured data to a site?
If you enable the RDF module, Drupal will automatically output some RDFa for your content. You can use a RDFa parser to extract it, e.g., http://linter.structured-data.org/. If it doesn’t find any data (under "Raw structured data extracted from the page"), you are likely using a theme that doesn’t support it. Use a default/core theme for testing, you should see it then.

If you want to provide additional structured data for your content types and field content, you need to map the content types to suitable RDF types, and the fields to suitable RDF properties. If you care about SEO, you’ll likely want to use the types and properties from Schema.org, as the big search engines sponsor this vocabulary, and use it for things like rich search results. 

One way to add these mappings is using the RDF UI module (which comes integrated with Schema.org).