Certification is a just extra curricular for your career...
Of course Certification will give you more opportunity...
But Certification is not all... You have to very much strong about you functional knowledge..
Certification can't clear you technical round in interview..
If you do Certification, then you can get more interview calls, but for technical round, you have to know more about functional knowledge...
As you want to learn MM module, so I can refer you the document to learn more about MM configuration SAP MM Configuration
Read and try to remember the all things which is explain in this document ...
It will be very helpful for you...