I'm learning D from 8 years in C++. My question is with regards to D garbage collection - when do I use delete, and when don't I need to?
You don't. Delete is not to be used with D version 2 and intended to be removed from the language. What the hold up is, I am not sure. Instead you use a function, destroy(object), which calls the destructor where you can free resources that are not GC memory. The destructor will be caused again during GC collection of the objects own memory. This is explained in "The D Programming Language".
The idea is to reclaim resources earlier than what the GC would provide and prevents memory corruption from dangling pointers. To be less safe the core.memory module provides GC.free(object) which can be used to free the memory, after calling destroy(object).
As I'm not a C++ programmer, I don't really know the RAII pattern, but this and reference counting is the expected strategy if you wish to avoid the GC.