The RHCSA (Red Hat Certified System Administator) certification is recommended for new system administrators. There are no prerequisites to the exam but it is recommended to have a few years prior Linux experience. Those who have taken the exam can move on to the RHCE certification.
Red Hat offers training classes for the RHCSA but there is plenty of free and payed training material available in print and online. Of particular interest is material from Dutch Red Hat instructor Sander Van Vugt on his website rhatcert.com. Some topics such as kickstart are no longer listed under the official objectives but still covered in prep material. As a rule of thumb, if it is not listed under the official objectives it won't be expected on the exam.
For the purposes of studying, CentOS and RHEL are identical. There is no reason to purchase a RHEL subscription to prepare for this exam.