Hello @dollydmk

Hope you are doing well.
To make you site with shop with associated add to cart first you need to have WooCommerce plugin installed.
If you don’t have you can download it from here

https://wordpress.org/plugins/woocommerce/

If you have already installed the plugin, the plugins will create Shop, Cart, Checkout and My Account pages automatically. Check the pages going to Pages section of Admin Panel.
Now you need to add Shop page to the menu that you have currently set as Primary Menu.
Now that should be done by now.

And further you can add product from Products custom post type that you will see after WooCommerce plugin is installed.

For more reference you can refer these documentation

https://docs.woothemes.com/document/woocommerce-pages/
https://docs.woothemes.com/documentation/plugins/woocommerce/

Hope this will help you.

Best Regards!!!