git init .
Then create/edit the .gitignore file and add the names of any files you don't want included in the repo (passwords, compiled stuff etc.), then
git add . git commit -am "Yay, my first commit!"
That sets everything up. Then when you make changes:
git add lib/some_new_library.rb git commit -am "Add SomeNewLib to repo. Fix a broken thing."
The '-am' parameters mean, respectively: automatically add modified files to this commit; use the last part of the command (the bit in "quotes") as the commit message.
You can make a new branch based on the current one:
git checkout -b fancy_feature
The -b means 'create a new branch'. Leave it out to switch to an existing branch. You can merge changes from another branch to the current one:
git checkout master git merge fancy_feature
This switches to the branch 'master', then merges 'fancy_feature' into it. If you edit the same part of a file in two different branches, then try to merge them, you'll get merge conflicts, which are a pain in the arse to fix, so don't do that.
To see which branch you're on, and what files have changed since last commit:
git status
To see what specific changes you've made since last commit:
git diff
You can include a filename after that to only diff that specific file.
Thus concludes my 5-minute intro to Git.