This (https://cran.r-project.org/doc/manuals/r-release/R-intro.pdf) is the little book that a lot of people used to use to learn R. It depends a bit on whether you want things from a more computing side or a statistics side. This covers both, but does not go into depth in either. Its free.
I would really recommend Coursera courses, I have done some myself. They have many R courses, including one that is for general introduction to R programming:
https://www.coursera.org/learn/r-programming
They have an "audit" option that is for free, you get all the material (videos, transcripts, assignments, etc) but you won't get an official Coursera certificate.