I started graduate school knowing absolutely nothing about R. Now, it’s my primary language and comfort zone. Below are a some of my favorite resources that helped me develop my R programming skills:
Best for the basics: Read these start to finish
R Programming for Data Science by Roger D. Peng
Stat 545 by Jenny Bryan
Best all around reference:
R for Data Science by Hadley Wickham and Garrett Grolemund
As you get more advanced:
Advanced R by Hadley Wickham
Other useful resources:
Getting started with Git and GitHub by Jenny Bryan
R Markdown: The Definitive Guide by Yihui Xie, J. J. Allaire, and Garrett Grolemund
The R Graph Gallery by Yan Holtz