You can start coding now and compiling later, you will need a minimum of a Text Editor - Text Wrangler is the best for OSX and its System 6/7 counterpart - BBEdit. You can write up the code and save it as text (.txt) file.

Now which environment to program in? C/C++ or 68K Assm? You can try to find MetrWerks Pro or you can use Symantecs C/C++. Codewarrior (6 Pro is the last 68K version) is another C/C++. There are others out there to look for.

You will also need ResEdit or Super ResEdit to built your resource forks.