Digital Tutors and Lynda will definitely teach you a few things, but from personal experience the hard way has been the best way for me. I just take every job I can, and when I run into something that I don’t know how to do I learn it!
I end up with almost every type of job coming across my plate and each one teaches me something new, it may be the most stressful way to learn but I think it’s also the best. Once you get the basics down (videocopilot.net) you just need to start going out there and getting jobs. You’ll learn new stuff on each project you do – one day you’ll need to do audio visualization, the next invisible compositing, the next a lyric video, etc.
And as for your questions about expressions – check out https://aenhancers.com/ though from my experience expressions only help make things easier/faster but are not totally necessary to being a pro in AE. (Don’t get me wrong though, I love expressions!)
– Spencer