Contact Us page is a core module of Drupal 7, provided with the fresh copy of Drupal installation.
You need to enable the contact module to make it work.
Go to the module page
Search for contact module, and enable it
Now go to the Structure, Here you can find `Contact form` in the listing. Clicking on it,you can create a system contact form and set up categories for the form to use.
Once clicked on Contact form you will be presented with the site wide contact form organization page. Where by default you will find `Website feedback`, and a link is given to set up your side wide contact form
Now you need to provide the link for contact us page
Go to the Structure->Menus->Main menu
Click on Add menu
In `Menu link title *` = Contact Us [its upto you what you want to see in the link title]
In `Path *` = contact
No contact us link must be available on you menu and clicking on will take you to the site contact us page