Visit this page and your download should start automatically. If it doesn't, click the indicated link to download manually.
Click on the downloaded file to open it in the finder. Then press Control while clicking on the .pkg file and select open.
Continue through the installer, selecting the default options.
Open the Terminal by searching for it using Spotlight in the top-right corner of the screen. If you already have a Teminal open, you will need to open a new one to use Git.
Note that if you are using a Mac OSX version prior to El Capitan there may be additional steps needed to install Git successfully. You can find these steps in the readme file included in the Git download. (Double-click on the README.txt file to open it.)
You can now use Git commands within the Terminal. For example, try running
If running the above command gives you the error
Agreeing to the Xcode/iOS license requires admin privileges, please re-run as root via sudo, then run
sudo git --version, type the password you use to log in to your computer, and agree to the license by following the instructions on screen. Pro tip: you can skip straight to the bottom of the license by pressing shift+G. After you've agreed once, you should be able to run
git --version without errors in the future.