To get started, you need to learn C# and .NET. AutoCAD up to version 2024 will require .NET 4.8 and C#6 ( I think). Version 2025 will require .NET 8 and C#12. Microsoft Learn has free resources for those. Start with C#.

After that, you need to learn the AutoCAD api. The only book out now that I'm aware of is by Anton Hiuizinga. It's on Amazon.