If you are currently enrolled or plan to enroll in a CNA training class, then your course instructor will help you contact the testing agency to schedule a date for the exam. This is one of the responsibilities of any CNA course instructor, regardless of the facility providing the program.
Some states have designated companies which administer the exam and evaluate the test results, such as Prometric. There are different locations you can choose from to take the exam. You are the one who determines the date of the exam as long as the test center is available on that particular date. As a nursing assistant program graduate you may contact the testing center and schedule an exam date at a convenient location and time.
If you have not received help from you instructor, my advice is to call the school or facility that provided your training and politely ask them for assistance. Also ask about the test center's contact information. If you can't get the information that way, you can contact your state's Nurse Aide Registry. A registry representative should provide all the information you need.