I've done exactly the same about two years ago.
AFTER learning C, the gold reference on coding on the Amiga is the RKM Libs volume. It's oriented towards C (the most used language on the Amiga by far), and it goes from basic to very advanced. I found it's an awesome read because it goes directly to the point explaining how to use the OS and how some OS internals work, with some included examples. It's actually included in the DevCD2.1 others mentioned but nothing like a good book read so I've actually bought the book too. By the way, don't let the size of it intimidate you (almost 1000 pages:)), you don't need to read all that stuff to code on the Amiga.
As a compiler for a not very expanded Amiga I'd choose SASC. But if you want the stuff you'll learn to be usable on other platforms GCC is a better option (but with this one you'll need an 030 at least and even that will be damn slow).
Cheers
José
