How do I make full width page?

create a new custom page template https://developer.wordpress.org/themes/template-files-section/page-template-files/page-templates/ – possibly froma copy of page.php, but with the necessary code added near th etop, and with the line with get_sidebar() removed.

to make it full width, edit the CSS based possibly on the output of the page template reference by body_class() if your theme has that function included.

detail depend on what theme you are using.

http://codex.wordpress.org/Forum_Welcome#Include_as_Much_Information_as_Possible

for theme specific details, please ask in https://wordpress.org/support/theme/causes#postform