install CentOS 6
yum update
yum upgrade
check the server time. sync the time to a time server if needed (service 'ntpd')
download and install Postgresql Server v9.1 rpm package:
rpm -Uvh http://yum.pgrpms.org/9.1/redhat/rhel-6-x86_64/pgdg-centos91-9.1-4.noarch.rpm
This should have installed the necessary server package: postgresql91-server.x86_64.