check this post to learn R from scratch:

https://www.analyticsvidhya.com/blog/2016/02/complete-tutorial-learn-data-science-scratch/

As a quick reference I have found this site to be particularly useful in terms of accessing basic code. 

http://www.statmethods.net

read this 

https://archive.epa.gov/nheerl/arm/web/pdf/irss_2.6.pdf

You should be aware that some instruction focuses on R as a programming language, and some more on the applied statistical aspects.

There are free online course that are good.  Some are listed here.

http://rcompanion.org/handbook/A_04.html#_Toc459550305

A few online books / websites are listed here.

http://rcompanion.org/rcompanion/a_09.html

If you are interested in performing statistical tests, and not learning much about R as a programming language, I recommend these.

http://rcompanion.org/handbook/

http://rcompanion.org/rcompanion/