Check out the Emergency Nursing specialty forums. We have several threads on the CEN and even a CEN support thread. I would say you need to focus on the NCLEX and obtaining RN credentials. Without RN after your name, specialty certification simply is not an option.
Next, worry about getting through your first year as an RN. The first year will be hard and confusing. You really need to figure out how to be a nurse before looking at taking specialty exams. Understand, you will have more than enough challenge learning how to be a nurse during your first year.
In addition, you need to take courses such as ACLS, PALS, and TNCC during your first year or so as an RN and really learn and understand this material. The CEN assumes that you are well versed with AHA guidelines and the trauma knowledge is all based on what is taught in the TNCC class. You will also need pedi, adult, psych, medical, and trauma experience as the exam will cover all types of patients and patient populations.
Finally, you need to understand the ER process, the triage process, and understand how patient care flows and is priortized based on the ER process and triage process as a whole. Teaching, education, disaster management, HIPPA, EMTALA, and transport/tansfer considerations are all areas of knowledge that can be tested.
So, slow down and get through this first year, then worry about certification.