Good question.

As Apache Spark is an open source project there is quite a lot to find online.

I would start with checking some (free) MOOCs on Coursera, Udemy etc.

There are also quite some websites which can teach you a lot (including the Apache Spark website).

Besides that: learning python and some scala

And books of course, still a valid option. Hard to give an exact answer on this as everybody has different preferences and background.

And of course: stack overflow and this community here!