Hello,
First, in order to have what you request, you have to go in your required Item Menu.
1. Go to Menus dropdown => Main Menu
2. Select your required Item Menu, click on Name to edit
3. Go to Products options tab
4. Reproduce this configuration from my screenshot :
Click on the image to see more details
For have the product description on your products listing, this will require a
custom via an override view.
Have a look on
this other thread
to see how to proceed.
Hope this will fit your needs.
Regards