Following are a few important modules that you should learn with SAP MM:
Inventory Management, Purchasing, CBP and IV.
Simply put, it all depends on your interest in a particular profile like if you are looking for your career as an end-user of MM functional Consultant then go with basic Nd learnt PR , PO cycle. And, If you want to be a MM functional Consultant then you should be completely skilled with all the implementation related to SAP MM.
If you want to learn SAP MM from scratch, then here's the right video for you:
If you are more into reading, then do check out this material provided by Intellipaat: