How to create and display user content submission form?
The create content pages are separate from Drupal's admin section. You can set up the permissions so users can add content of a specific content type but don't have access to any other content types or the admin section. 

There's also a permission 'View the administration theme' if this isn't set for a particular role, but they can add content, then the Create Content page will use the default theme. There's also a setting at the bottom of the appearance page, if you deselect 'Use the administration theme when editing or creating content' then everyone who accesses the create content pages will use the default theme also.