I would begin by reading the following:
This will give you an overview of cluster suite. You will need to have your two PC's on the same network / subnet. If you want to share storage, read up on GFS or GFS2:
To get yourself going, install the following on both RHEL 5 machines:
yum groupinstall Clustering "Cluster Storage"
The above will download the relevant packages for you (assuming you are connected to the internet and have a support contract for Red Hat Network)
If you haven't paid for a support contract with Red Hat, I would recommend installing Centos 5.2 or 5.3 on your two machines (Centos is a RHEL binary compatible Linux distribution, no charge and you get all the updates and applications / cluster suite / GFS etc. for free).
In order to set up and maintain your cluster, one of your two machines will need to be running the luci admin software. Choose one machine then do the following:
you will be prompted to enter a password (twice). Once complete you can access the luci GUI by a web browser:
(where nnn.nnn.nnn.nnn is the IP address of the machine running luci)
Login to luci and then have a browse through the various options.
It's difficult to say exactly how you need to configure your cluster as there are many options. You could, for example, use GFS / GFS2 to share your storage if it can be directly attached to both machines or you could use GNBD / NFS. I would read up on the Cluster Suite and GFS / GFS 2 docs to familiarise yourself with the technology before formulating a plan of how you would like to implement your set up.
There is a lot to learn, I won't pretend that there isn't. The key is to read as much as you can, have a play and be prepared for the odd set back or frustration. You can always come back to the list when you need more help on a specific problem.