If you are looking for a page ID or post ID, please follow the steps below.
ID is a unique number for each page or post, no matter if is draft or published. To get proper ID, please go to the WordPress list of pages or posts (select right option in main WordPress dashboard menu on the left side). Then, please hover over Edit button of post or page you are interested in and in the left bottom corner, you will see the link. The number after post= is the ID you are looking for.
The same ID, you can find in main URL of browser when editing specific post or page. In this case, you have to be in edit mode.