Here is a collection of external resources for the tidyverse/ggplot2 and principles of good data visualization.

R, ggplot2, and the tidyverse

https://www.tidyverse.org/learn/ has a collection of links on learning R and the tidyverse packages
David Robinson’s Introduction to the tidyverse Data Camp online course
The book R for Data Science (complete version online available), co-authored by Hadley Wickham, the inventor of many tidyverse principles and packages.
RStudio Cheat Sheets

Data Visualization

Kieran Healy’s book “Data Visualization for the Social Sciences”. Many of this course’s exercises build on this excellent book.

Help, my R code doesn’t work!

Stack Overflow is your friend. Usually, every problem you encounter someone else has already encountered and asked about on this site.
#rstats Twitter hash tag. Often a resource for help and/or new package announcements; generally nice people are on the channel.
https://community.rstudio.com/. A new website, specifically designed for problems and questions regarding the tidyverse packages.