How to start Windchill data migration or Customisation

Take a look:

• "Windchill Training": https://www.ptc.com/en/ptc-university/product-training-catalogs/windchill-training 
• "Contact the PTC University Sales Team": https://www.ptc.com/en/ptc-university/contact-us 

I might recommend a course in Java, XML and JSP. 

@AL_8469056 Data migration and Customization are two different big Topics for Windchill PLM!

First of all, if you are a newbie in Windchill PLM, you should learn Windchill PDMLink Business Administration, we mean by it the functionality of the system. PTC has a full yet complete training catalog: see Windchill Training Catalog (ptcu.com), those are the live sessions on PTC Learning Connector.

However, there is no such full complete catalog for Windchill PLM Customization, and before dealing with customization, you should have knowledge in Java as @avillanueva mentioned, also check those posts:

• Re: Windchill Customization and Development Traini... - PTC Community
• Re: What stack is Windchill running? - PTC Community 

As to data loading, this is another topic, it's usually more business or configuration than customization, unless if you are dealing with the Bulk Migrator, custom loader, or Java APIs:

All data loading tools except one are collected in this illustration: 

Unfortunately, the above illustration needs a session to be explained!