If you're interested in developing for Amiga Anywhere/AmigaDE,
all you need to do is purchase an SDK from Amiga and get coding! If you get into
it a bit more, signing a Non-Disclosure Agreement with Amiga gives you access to
more files and as yet "not-for-public-release" code.
Amiga also offer an optional Software Distribution Agreement where you
don't even need to worry about how to market and sell you product as Amiga do it
all for you.
Of course, you may want a little help in coding for Amiga
Anywhere. At present, you can code in C, C++, Java or VP - the Virtual Processor
Assembly Language.
VP is an amazing assembly language with all the power of "real" assembly
languages, but due to being written for a "virtual" processor, supports an
infinite number of registers, high-level language style loops and generally
eliminates everything that makes assembly such a hassle to code normally.
Nextgen
Developments have an excellent tutorial section for coding in VP including
GUI applications.