I recently needed to install git on a CentOS 5.5 system. Unfortunately, the standard repositories do not provide the git packages. However, this is quickly fixed by adding the EPEL yum repository:
rpm -Uvh http://dl.fedoraproject.org/pub/epel/5/x86_64/epel-release-5-4.noarch.rpm
Then simply run
yum install git
Have fun with git!