We’ve all started at some point, so don’t worry about being a beginner. 
I think the best way to learn a new framework is just to get your hands dirty:

be an active member if this discussion board: have a look at some questions here and try to answer them (if possible with a small code sample showing the solution)
also working on some issues helps a lot as you will get familiar with the implementation (chose a simple one at first and have a look at the tests of similar methods to get a feeling of the new feature)
try to implement some papers in PyTorch and compare the results to implementations in other frameworks if available