This is an example of a full width page template.

You can choose a page template when creating or editing a page.
You can find the page templates on the right side in the field “Page Attributes”.

Screenshot: Choosing a Page Template

In your theme settings page you can define your default page template.
For this, go to to your “Theme Settings” to the tab “General Appearance” to the point “Sidebars”.
This setting is your global setting.