You only have to set this up once per machine.
From RStudio, go to Tools > Shell and tell git your name and GitHub email by typing (use your own name and email):
git config --global user.name 'Bernhard Konrad'
git config --global user.email 'bernhard.konrad@gmail.com'
These commands return nothing. You can check that git understood what you typed by looking at the output of git config --global --list.
Resources: