If you do not wish to show Sidebar than choose No Sidebar Content Centered on the particular page, just edit the page and choose the No Sidebar Content Centered from Select Layout Attribute Box on bottom right. For a particular page, if you select the Layout in the page than it will override the Layout setting set from the customize.
If you wish to change the Layout of all the pages than go to Appearance-> Customize-> Design Options and choose Default Layout For Pages only.