How to insert html tags to a node content?
To use html tags in the Body field of a Basic page content type (for example) you need to select "Full HTML" or "Filtered HTML" in the "Text format" dropdown menu below the input text box (see image). As far as I know, only fields of type "Long text and summary" and "Long text" allow you to select Full or Filtered HTML. 

To configure text formats and permissions go to Configuration -> Content authoring -> Text formats (www.example.com/admin/config/content/formats). 



For more information see Text filters and Input Formats.