To me, starting off with OpenGL might be a bit to much.
But since OpenGL is a standard on several systems, your code (made on the PC) should rather easy to port.
But befor you start doing that, might I suggest starting off with some amiga specific programming.
Anyway, talkin about C (ANSI), then there are no big difference, I have been programming PCs amigas and Microsystems for some time now, most in ANSI.
And the code is rather portable, ofcourse, there are some system specific functions. that you need to replace.
Coding Demos using C should not be a problem, and as for speed. C compilers are very good, and on an A1 I don't think speed should be your biggest concern.