Hello. Glad to see you wanting to expand your skills with Dynamo. Python is very useful with Dynamo and even outside of Dynamo/Revit.
As for whether you should learn Python using Revit as the context or separately, that is up to how far you want to go with Python. A good groundwork of Python understanding is best for both cases but after that, learning just in the context of Revit is just fine.
For basic Python, a coding tutorial website would probably be the best, of which there are plenty of. Something like codeacademy.com, etc. (not affiliated with).
Here is another thread where a list of resources specific for Dynamo are compiled, check it out: Python Nodes Basics