Presented here are a number of online resources for people interested in learning how to program in R. Although I have only included resources that look to me to be of relatively high quality, I have not reviewed all of them as extensively as I would like. Use at your own risk...
A tutorial for learning R, which I developed for my Marketing Analytics course at RSM
An online R course by Paul Hiemstra
Learning R and R Studio with an emphasis on the latter
Documentation for the R Markdown document format, which is a mix of descriptive text, code, and the output of that code when executed by R
Materials for a course about R, RStudio, git (source control), etc.