Queries regarding SN Certification

Hi,

I am using Service now from 2015 in which I am the CMDB (HAM & SAM) caretaker in my company.
Have basic queries regarding certifications, Please help.

1. Do I need to take CSA Ceritifaction or can I directly take CIS - PA Ceritifaction ?
2. Since I take care of HAM & SAM Do I need to take CIS ITSM as overall or should I must take CIS - HAM & CIS - SAM ?& What is the prerequisite for to take up this exam? is CSA Mandatory?
3. For my profile which Certification holds good. Please help...

Thanks
Sunil Shivaram