I know not many people know Spanish round here, but I know for a fact https://www.domestika.org/es/courses/39-motion-graphics-y-diseno-generativo this course is pretty good.

If he ever stops responding to questions in forums, I'll stop knowing how to do after effects expressions, really.

I've heard good things about this series:
http://www.provideocoalition.com/after-effects-extendscript-training-complete-series/

I picked a book recently called simply "After Effects Expressions" by a guy called Marcus Geduld. It starts with very basic examples and gets increasingly complex, but he never lets the writing get too dry so it's very easy to follow. I came at it as more or less a complete novice and it's definitely improved my abilities with expressions.

Keep in mind AE Expressions are a subset of JS and lots of modern JS learning resources will be written with someone working on Node and frameworks in mind and tend to over complicate things. You only need to know the absolute basics of the syntax to work with AE expressions.

I've been using Khan Academy (https://www.khanacademy.org) to learn JS. It's awesome, it's free. You may enjoy it if you're not in a rush to learn and, like me, need someone to explain it to you like you were 5 years old.

I second Greg regarding Dan Ebberts. He's brilliant and I can't thank him enough for all the times he either answered me directly via forums and helped others for the past 15 years. He's the dude...
https://leaders.creativecow.net/leaders/ebberts_dan/

If you’re altogether new to programming, I’d pick up a book on Javascript. O’Reilly makes great ones: https://www.amazon.com/JavaScript-Definitive-Guide-David-Flanagan/dp/0596000480

http://www.w3schools.com/js/ is another great quick reference for the language. They also have tutorials you can go through and tinker with.

Specific to After Effects, I’d start with http://www.motionscript.com/. Dan is brilliant and breaks down complex expressions into easily digestible pieces.

There’s also AE's Expression Reference.
https://helpx.adobe.com/after-effects/using/expression-language-reference.html

Start simple. Think of a basic problem and search for ways to solve it. Find and deconstruct other expressions and re-work them for what you need.