You can begin learning SAP mm from online sites for getting the theory intact like Intellipaat blog on SAP MM. There are several blogs that explains the concepts and understanding on how mm works. Websites such as SAP administration / development information, news and tips provides highly-level overview to establish basic concepts and understanding.
If you are someone who already studied SAP MM then take a look at the Interview Questions to get an idea of the questions you may face in an interview. https://intellipaat.com/blog/interview-question/sap-material-management-interview-questions/
Of course, you can add a feather in your hat by getting certified in SAP MM.