Easiest way is to download and run pre-compiled git installer. You can get it from http://code.google.com/p/git-osx-installer/.
Another way is use MacPorts http://www.macports.org/install.php.
Run following command to install git using MacPorts.
sudo port install git-core +svn
If you need more information how to use git, I recommend this site – http://progit.org/ which contains whole content of book ‘Pro Git’.