RProject.org and RStudio.com Get started by installing R (all for free)! First install the latest version of R itself from RProject. Then, install RStudio.
RStudio Cheat Sheets R basics in easily accessible and laidout format! Heck, print it and tape it to the wall above where you work.
Using R for Introductory Econometrics by Florian Heiss. This is a free ebook (physical version available cheap on the website) that describes using R in introductory econometrics courses. The book is designed to work alongside the introductory Wooldridge textbook.
Guide to R for Santa Clara University Economics Students.
Extensive and very useful guide taking you from the very first steps of using R, with an economics focus. Holds your hand!
My Introductory R Script. This is the R script I used to hold the R training workshop. This is the same script that many of your faculty saw, so there's a good chance they may be basing the code they expect from you on this script! This will walk you through the basic estimators you're likely to see, as well as manipulating data and getting started.
Introduction to the dplyr package. If you have your own data that you need to clean, the dplyr package is indispensible. Doing things like subsetting data, renaming variables, creating new variables, and sorting data are certainly possible without it, but much easier with it! Watch this video!
Introductory Machine Learning in R. Most of you won't need this! But if you're interested in machine learning, R is the place to go (unless you know Python), and you can get started here.
Graphs in R (ggplot2). More advanced stuff. But if you want to make some real pretty graphs in R you have to learn ggplot2. Here's a place to start.
R for Stata Users. Making the switch? I've found this site very helpful, and a good source of functions that I've literally spent hours trying to replicate! Also a good source on more info for using dplyr in the "SplitApplyCombine" section.
