Download git here: http://git-scm.com/download
Set up an ssh public key and register an account at github.com: http://help.github.com/mac-key-setup/ (apologies to non-Mac users)
Enter the following at the terminal (with obvious substitutions):
git config --global user.name "Your Name"
git config --global user.email email@example.com
To pull a repository called "repo" from user "person":
mkdir repo (or what have you)
git remote add origin firstname.lastname@example.org:person/repo.git
git pull origin master
In particular, every call of "git pull origin master" pulls the currest master version off github. Be sure you're working with the current version before trying to push local changes, or you might get conflicts. To commit changes, either call
git add files_that_changed
git commit -a
which commits all changed files. Committing brings up a text editor where you describe any changes made. Be aware, if you don't write anything the commit will be aborted. Then to push your changes from the local machine to github just type
and that's it! Things get messier when working with branches, checkouts and merges, but for 90% of what I do the above suffices. The web abounds with tutorials and a quick reference sheet can be found here.