This article explains how you can get started with WHMCS after installing it.
If you have not installed WHMCS, you will first need to make sure that WHMS is available via "Site Software" in cPanel for installation. The following article explains the process.
Once done, you can install WHMCS via the information below.
Please note that WHMCS will need to be installed on a cPanel account using a domain you have registered. It will also need a DNS A record that points to the cPanel server in question. Once the installation is done, WHMS has a very useful guide for getting started.