How to Add Sidebar for a Page

Add default Sidebar

Step 1: Go to Appearance >> Customize

Step 2: Find Sidebars

You can change General Setting, Single Sidebar Layouts, Dual Sidebar Layouts here.

Add Sidebar for particular page

Step 1: After creating a page, come to Page Options

Step 2: Find Sidebar

Choose Sidebar and position you want to use and Hit Publish.