Drupal Commerce default amount before adding a product I want that the Drupal Commerce module checkout by default starts with one specific product , whether you choose another product or not.

Is this possible to achieve this by building a rule? Which rule do I need to build?
You can do this, but I doubt that this is possible with rules.

What you want is a custom checkout flow. Either this can be done by altering the checkout flow provided by Commerce, or disable it completely and write your own.

Either way you will need to write your own code to create this custom checkout experience, and this is beyond what Rules is capable of.