Tim's guide to using GitHub with RStudio
Short guide - some steps missing http://r-pkgs.had.co.nz/git.html
Extensive guide - with less emphasis on RStudio http://happygitwithr.com/
Great (but long) video from Jenny - talks more about RStudio https://www.rstudio.com/resources/videos/happy-git-and-gihub-for-the-user-tutorial/