I would say find a SQL tutorial to begin with, in the data load editor everything below starting at Select is basic SQL and the Load section will be Qlik specific. The only thing to remember is Qlik will do it's own joins. You might look at Mode Analytics as they have a free SQL tutorial that will give you some basics. I would also suggest looking at Udemy, I would pay attention to reviews to make sure it's a decent course, but there are several on Qlik Sense and they run like $10.99 sales all the time. The last thing is use the Qlik resources, the Video, documentation, and this awesome community! Best of Luck!
I suggest starting with a book. There are a lot available around the Qlik environment: Books and literature and here I recomend:
QlikView Developers (June 2017)
Barry Harmsen und Miguel Garcia
This book is written for View and not Sense but from the datamodelling and scripting there are no differences and IMO it's the best book to start and to understand the logic behind the tool and BI. You could do all the stuff in Sense too, you just missed the opportunity to use the qvw's with the various intermediate steps and final solutions (unless you install View in parallel).