wow! your going a bit fast!!
dont try to learn too much of a language at a time!
you will probably notice that in the course your doing they will teach you in small steps, thats the best way!
if your C code is plain ANSI,( or even K&R), the exact same code should work on all computers with an ANSI C compiler!
get a C compiler for you amiga, and use it to practice on!
take home examples from the course, and use them for practice at home on your amiga!
the only problem is, they may use some includes such as "console.h" - just take that out & dont use any of the functions defined inside, they wont work on the amiga
AmigaOS never had very good support for C++, the standard way to code on amiga is C, not C++, you can still use C++, but by default you will have to call the os functions using C, unless you find some third party C++ libraries
dont worry about demos etc... for now, concentrate on the _language_ first, then learn some of the basics for intuition programming, just to get a feel for amiga programming, writing graphics software for the amiga, is a hell of a lot more advanced than just plain vanilla C, which, can be hard enough to learn on its own!
:-)