Hello there Dennis, and welcome. Firstly I must say Iím no expert with clientexe however a good place to start is here: Clientexe Getting Started
Where you will find the manual and instructions with a mixture of tutorials with getting started and what and what not to do. As for the order form and setting up accounts clientexe will do this for you after you have setup and created your packages (both on Webhost Manager (WHM) and clientexe) and changed a number of settings.
For the Credit card (CC) processing this will most probably be handled via PayPal, MoneyBookers or Google Checkout for example and there are options provided within the admin panel of Clientexe to enable and disable this (as well as entre the account details for these accounts).
Personally Id always recommend seeking out an SSL (secure Socket Layer) certificate to add peace of mind, trust and that extra security to your billing system, as you are still governed by DMCA (copyright act) and others for the safety and usage of personal data despite you most likely not having access directly to the card or bank details of a client.
Hope that helps