what we need to learn to go
TypeScript and ES6
HTML & CSS
Since, we are talking about building frontend apps, knowledge of HTML and CSS is necessary. So, you would be creating your template or view using HTML and CSS.
Next most important thing that you need to understand is about the angular versions. Basically, angular follows semantic versioning approach for releasing new versions of angular. Unlike, AngularJS all these subsequent versions are back compatible and are similar in terms of code base (except few deprecations and changes).