The main page of R-Project: https://www.r-project.org/
RStudio, a very good user interface, editor, and development environment: https://www.rstudio.com
Info in Danish: https://logistikforum.org/r-project/r-project-information
Advanced R Book: http://adv-r.had.co.nz/
Quick R: https://www.statmethods.net/
R Programming/Mathematics: https://en.wikibooks.org/wiki/R_Programming/Mathematics