To begin with, I would recommend that you learn the basics Qlik Sense Home ‒ Qlik Sense. A good addition will be the following books:
1. Learning Qlik® Sense. The Official Guide. Authors Christopher Ilacqua, Henric Cronström, James Richardson (first published: February 2015).
2. Qlik Sense® Cookbook. Authors Philip Hand, Neeraj Kharpate (first published: November 2015).
Although the books are written on the basis of older versions Qlik Sense, they contain many useful ideas applicable to any versions of Qlik Sense.
Then, your practice will show which areas you need to study further.