To set a minimum order quantity for a product go to Products > View Products menu. Once clicked on this will display all products available in your store.
To edit a product click on the "pencil icon" next to the product.
The product screen is split into tabbed sections: General, Specifics, Connect, Features, Variants, Images, Promotions, SEO. Click on the tab called "Specifics".
Next to the heading "Minimum Quantity" you will be able to set a minimum order quantity for your product.
This is the minimum amount of the product the customer must purchase to add the item to cart. e.g. If you want customers to have to purchase 3 of your items as a minimum set 3 as the number.