Hello Bala,
So you know, "Cluster Mode" is no longer the term we use. We now use Data ONTAP or just ONTAP. I would recommend that you start with the NetApp Certified Data Administrator, ONTAP certification. You can find more about this certification here. To help prepare for this certification, I would check out this page which lists the recommended steps to help prepare you for the exam. This includes NetApp approved training courses, a Reference Document which lists documentation, white papers and other sites that can be referenced to help prepare you for the exam, as well as a practice test. Then, when you are ready to take the exam, you can go to Pearson VUE to register for the exam.
Once you pass the NS0-158 exam to earn your NCDA ONTAP certification, I would recommend looking into the NetApp Certified Implementation Engineer certifications, either the NCIE SAN ONTAP or the NCIE Data Protection.
Please let me know if you have any additional questions.
Greg Hyman
NetApp Certification and Accreditation Program Manager
ghyman@netapp.com