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.