The most important thing in all programming, including creating visualizations, is to know what needs to be done and why. How to do it will come naturally to you with practice and tons and tons of Google searches. Specifically regarding a starting point for visualizations, my advice is to pick a language or tool you know enough, and learn visualization theory; a lot of it is a guessing game and personal taste, but there are strict rules that apply to information design that don’t apply to other types of design. Once you have the what and why, you can bounce around different tools and languages as necessary and will have a better idea of what you need to be focusing on learning as you add more tools to your kit.
Udacity has a Data Visualization and D3.js course that I really like. Last I looked, all the code was long since deprecated, but there are non-code units that discuss chart types, visual encodings, bias, and context that were all stellar. They feature Cole Nussbaumer Knaflic of Storytelling With Data pretty heavily. While she focuses primarily on the type of explanatory visualizations that a person can glance at and understand within seconds, it’s a really good place to build up your foundation. They also have a Data Visualization in Tableau course that I know nothing about and can’t vouch for, but if you want to try out Tableau, maybe look into it.
I also really like Alberto Cairo’s Visual Trumpery lecture (I promise not all the examples are political and the ones that are are bipartisan and even international, the title is intended to be provocative) where he talks about graphicacy–graphic literacy–and misleading visualizations.