You can go through the information in the following URL and then some of the resources like webinars, ebook & whitepaper listed there.
https://www.servicenow.com/products/it-operations-management/what-is-cmdb.html
Also check the latest CSDM white paper to understand the common service data model.
It is a vast area with lots of topics and subtopics. But still you can try to get a good understanding of the basics by exploring the following on the instance:
1.Understand the attributes and data of various tables present under Configuration>base items module and CSDM module.
2. Go to cmdb_rel_ci table to understand about CI relationship types and how they are defined.
3. Open CI class manager to get a view of the CMDB hierarchy. Then click on some of the tables to understand how properties like identification rules, reconciliation rules, cmdb health metrics etc. can be defined for each class.
If you have access to log in to nowcreate, you can get all sorts of useful CMDB & CSDM artifacts (like process guides, workshop decks) for free from https://nowlearning.servicenow.com/nowcreate
