To add to this: I would start with basic analogue electronics as it forms the basis of everything else on this list. I found https://courses.edx.org/courses/course-v1:MITx+6.002.1x_1+2T... reasonably good for this.

Learning analogue fundamentals, learning to read a datasheet (probably one of the most important skills for real electronics design!), and C for embedded systems are probably the real keys here.