Phantom :

The given answers represent excellent advices : To port programs you need a general knowledge in programming and you could start with simple SDL examples.

If you want to port programs from UNIX you also have to understand some UNIX specificities.

Last, I wrote a document that could be useful for you to learn some things about the differences between the several Amiga systems, explaining how to write portable code : http://main.aminet.net/dev/cross/PortableCode.lha