Before touching any customization, get oriented: what the platform is, how it's organized, how to find your way around, and how to set up an environment you can actually practice in.
What the platform is, how it relates to Dataverse, and how it's different from an ERP system.
What an environment actually is, and the difference between sandbox and production.
Apps, the site map, records, views, and forms — finding your way around before you customize anything.
What a license actually controls, explained conceptually, without chasing exact prices.
Getting a trial environment, and the tools every Dynamics 365 CE developer keeps close: Visual Studio, the Plugin Registration Tool, XrmToolBox, and Postman.