First set the 'Price' of the product so that it is lower than it's 'Compare at price':
To display the price savings on your product page, go to Customize theme
Online Store > Themes > Customize Theme
In your shop preview, click on a product. This will bring up the Product tab.
In the Product tab, you have many display options, including the option to 'Show price savings':
To display the price savings on the Cart page is very similar.
In the Customize theme shop preview, click the cart icon and the Cart page tab will appear with your display options:
There is also an option to display the price savings on the mini cart:
Display the savings on the mini cart are set by going:
Online Store > Themes > Customize > Header > Theme settings