Everyone should read the first three sections of this tutorial by Karl Broman, a statistician. He uses the command line version of git. In RStudio most commands translate directly to buttons: push, pull, diff and commit. status is mostly conveyed by the git tab in RStudio. You can always use the command line version, by clicking the cog and then “Shell” in the git tab in RStudio.
Git Basics This is good to read to get a feeling for git’s model. Pay particular attention to “The Three States”.
http://toroid.org/ams/git-central-repo-howto This is good to read about how merging and conflicts work in git particularly the “Pushing and pulling section”.
RStudio has a bit more info on getting a set up with git: Using git with RStudio
The git site has very thourough documentation: http://git-scm.com/documentation
Github has lots of links for learning more: https://help.github.com/articles/what-are-other-good-resources-for-learning-git-and-github
If you want to learn more command line git usage (i.e. not in RStudio), start with: Try git.