Many external resources are available in order to teach you how to develop with Orchard Core.
Lombiq created a Demo Orchard Core module for training purposes guiding you to become an Orchard Core developer:
The module assumes that you have a good understanding of basic Orchard Core concepts, and that you can get around the Orchard admin area. You should also be familiar with how to use Visual Studio and write C#, as well as the concepts of ASP.NET Core MVC.
Lombiq publishes a weekly newsletter to inform the community around Orchard with the latest news about the platform: