Hello,
The best resource to learn Scala language in my opinion is Martin Odersky’s two (Principles and Design courses) Scala courses available in Coursera.
I wonder if any one teaching Scala better than the creator himself. Martin is also a professor in Ecole polytechnic in Switzerland so he knows how to teach.
Once you finish these two courses then based on your interest you can start learning Spark or start developing applications or micro services using the language. In the latter case you may need to learn cats library but not necessary.
I can say that I started loving this language after completing the two courses mentioned above.
One last note. Taking those two courses is not an easy ride but it is definitely worth it.