It is not going to be easy... NPrinitng is an enterprise level tool. You need a NPrinting license and a server to start with. For simplicity you could use QlikView as data source, but you need license for this too...or Qlik Sense Enterprise server (which again requires license)... So that is starting point and first question: do you have NPrinting License and do you have Qlikview or Qlik Sense server license?
if the answer to any of this questions is no it will be hard for you to learn.
I will add to it - in order to learn NPrinitng well you should be an advanced QlikView or Qlik Sense developer to say the least... 90 % of planniing and designing starts there so it is closely related factor to proper nPrinitng development.
The other thing is that unless you have an environment you can practice and constanlty work on there is no point for you to learn in advance as many things change constanlty and you need to be constantly up to date on what new features are or how you can go about certain things.
Lech is right, it is not going to be easy at all. This is a difficult product to use and you must have a solid base of Qlik knowledge. You have made a good first step in coming into this community. There is a wealth of videos, documents, discussions, and very talented people that can assist you. I would start with Lech's recommendations of making sure you have all the necessary licenses . Then you will need to decide what you want to learn first. The links shared by Lech and Giuseppe are great starting points. Then read a LOT and watch a LOT of videos! I am learning something every single day in this product and don't see that ending anytime soon. Don't let it intimidate you! just start small and simple and build your knowledge.
What is your intended use? Do you have a report style or an end goal or need in mind in learning NPrinting?