See postgres installation instructions at:
This contains instructions for all 7.3.x versions. Also make sure you do not already have postgres installed. It comes with RH 8 and 9. There are some issues to content with if you are trying to upgrade from the stock version to 7.3.x. You can check to see if it is installed a lot of ways. One may be to run:
find / -name postgres
if you get a result, it is already installed. Then you just need to learn how to use the version you have unless you need the latest edition.