Are you really running OS4 on your A1200? If you are, then I know nothing about the software side but I would have thought you have more than a passing knowledge of the Amiga :-D
To get on the net you need hardware and software. Hardware should be a compatible PCMCIA ethernet card or wireless card. Well, I say wireless, Prism2.device is what you need for wireless but I don't know if it works on OS4.
Your best bet is to check OS4Depot to see what software you need. You need a TCP stack which I am sure is distributed with OS4?