To be transparent with your users about what personal information you collect and what you do with it, you are required to publish a Privacy Policy agreement on your website or give in-app access to it.
Websites usually post a link to the complete Privacy Policy agreement from the footer of the website, whereas apps generally add the Privacy Policy to an "About" or "Legal" menu.
Another popular location for ecommerce store apps and websites is the checkout page, or account registration page if you don't have an ecommerce component but allow users to create accounts.