1. Download Git from the OSX link:
http://git-scm.com/download
That will take you to this Google Code project:
http://code.google.com/p/git-osx-installer/downloads/list?can=3
2. Run the Installer.
3. In order to allow someone to clone your repository, you need to make sure that git is part of the system PATH.
Create or Edit a file called ~/.bashrc
Add the following:
export PATH=$PATH:/usr/local/git/bin
Now verify that the path is accessable to a no-login shell:
ssh localhost echo \$PATH
You should see something like this (Just make sure /usr/local/git/bin is in it)