Newly PO - Doubt about how to manage User Stories working with design and development teams

I am a junio PO

There's no such thing. You're either a PO or you're not, with a product to own and value to maximize. In your case you appear to be something else, because you're trying to define a workflow for Developers to follow. In Scrum it would be up to them to self-organize their own way of working. 

As a Product Owner in the Scrum framework, you are not responsible for the development workflow.  You are however responsible for clearly defining your Product, enumerating all that needs to be done to the Product to keep it viable, and making those items visible to anyone with interest.  You are responsible for building relationships with your stakeholders so that you can represent their interests.  You are responsible for ensuring that the Scrum Team focuses on the work that provides the most value at the current point in time. 

I suggest that you read the Scrum Guide, especially the section that explain the Scrum Team and its three Scrum Responsibilities.  This link will take you the online English version.  If English is not your preferred language, this link will take you a location where you can download a copy of the Scrum Guide in a language with which you are more comfortable. 

I am a junio PO with no much experience in Agile/Scrum

To echo @Ian, there is no such thing as a junior Product Owner in the Scrum framework.  I also want to point out that Agile and Scrum are not the same thing.  Scrum is a framework to help organizations be more agile but there are many ways to be agile that do not involve the Scrum framework.  Since it seems the organization you are in is new, it would be a good idea for you to understand the differences and then work with the rest of the people in the organization to determine how they would like to proceed. Just using words from the Scrum framework does not mean you are following the framework.  And many agile methods, frameworks, techniques use similar terms but they do not always mean the same thing. 

Hey guys!

Thanks a lot to both of you for your answers

I will read the guide, and actually that is what I am doing, trying to understand the whole framework.

When I said I am "junior PO" I meant I am trying to become one by educating myself and later on get the certification but in the meanwhile I have to create all the processes for my company. (Actually I am enrolled for the free SFC certification, to start with)

I understand that the development workflow is maybe not reponsibility of the PO but I am going beyond Development workflow. We need the product end to end workflow to be tracked and to be followed (and this will include the parts of the development) and the idea is to use scrum framework (sorry If I am saying something which makes no sense)

So if PO is not responsible to set up the end to end workflow, then who is it? And then who "controls" that the activities agreed on are being done and the sprints goals are being met? 

Thanks agains for your help

So if PO is not responsible to set up the end to end workflow, then who is it? And then who "controls" that the activities agreed on are being done and the sprints goals are being met? 

In the Scrum framework, the Scrum Team is a self-organizing, self-managing group.  This means that no one "controls" and everyone is responsible.  There is no hierarchy in a Scrum Team. There is no leader.  The Scrum Team shares a common purpose and create common goals.  The way that work flows is decided on by the people doing the actual work.  The Scrum framework is successful when an organization is willing to let the people closest to the work make decisions on how to do the work and what work is most needed at a specific time.  

The Scrum Team takes pride in the work they do and will strive to deliver value in every Sprint. The Scrum Team hold each other accountable for "the activities agreed on are being done and the sprints goals are being met" because they value each other and the contributions made by all. 

I caution you about the certifications you choose to pursue.  Many free certifications may not be based upon the Scrum Guide and will teach you a "Scrum" that is not true to the Scrum Guide.  I have attended multiple Scrum Fundamentals Certification classes that my employers have provided.  None of them have been the same and all of them "taught" certain practices that are not part of the Scrum framework.  I suggest that you use the resources provided by this site if you want to learn Scrum.