Add new page

The question posed has nothing to do with a theme as content types are independent of theme in use. Please edit your post and move it to the 'post installation' forum. Thank you.

There are three important components that you have to keep in mind while creating a page.
Regions - the various sections of a page
Blocks - can be placed inside a region and used to expose some content
Content - piece of information that you generally create basis of a defined content type

Now for the query about contact us. This page already exists on Drupal you just need to enable it.
The about us and other pages you can create as new pages and link them to some menu items which you can create again from structure admin menu.

Thanks!

Use the "add content" feature from admin environment to create new nodes. There are built in article and page content types. Create them. Give each a URL. You then have pages.

You can also define your own custom content types with additional fields if you like.

For some examples of ready-made Drupal 8 sites, check out something like the following commercial Drupal distributions. They have free ones, but the paid distributions include custom content types and content, sort like an advanced "drupal website in a box", and well worth it.

http://www.morethanthemes.com/drupal-8

For a specific example, check out the demo of their Drupal 8 "enterprise plus" distribution. You can learn a lot about Drupal just by studying their products, even by studying just the demos. The demos provide a representation of what Drupal is capable from a UI perspective (essentially limitless).

http://demo.morethanthemes.com/enterpriseplus8/default/