I am not sure if there is an official certification for a "Marketo Super User". What you can do is take at look at these certifications:
Marketo Certified Expert: Marketo Certified Expert Preparation Tools
Marketo Champion: Champion Application
Marketo Certified Revenue Cycle Analyst: (going to be retired soon)
Marketo Certified Consultant: Marketo Certified Consultant Preparation Tools
Go to University to learn more about the system overall.
Learning by doing is how most of us do it.