Please note that CentOS, like other RH derivatives, uses sendmail by default.
Run the utility system-switch-mail from the command line as root (you may need to install it with "yum install system-switch-mail), and it will set a range of system variables and symlinks for you (ie make postfix the default in place of sendmail). This step will take you about a minute and will save you a lot of grief.
Aside from that, start at the howto section at
www.postfix.org