Make an attribute "Required"

Brand new zen cart noobie...

How do I go about setting the cart to give an error message if a color or size is not selected?

Other words, if a user chooses a shirt but does not pick a size or color, they can still add it to the cart, I want to make sure they have to pick a size and color, or whatever attribute is set for any item... is that possible?

:lookaroun

In Catalog > Attributes Controller, when adding/editing an attribute, either make one of the sizes/colors "Default", or add another attribute for the set named "Select" or whatever you like, and make that Default and Display Only. This will force the customer to actively make a choice; the first version is good where most people choose one particular option.