Sadly, iChat does not *yet* have profile capabilities. In order to post your info for others to see, you need to download a program called iProfile. A link should be on the forums somewhere. The program is no longer being developed to my knowledge, but should still work just fine (I use it and it does what it's supposed to).
The ability to do this will be integrated in Mac OS X 10.4 Tiger, with iChat v. 3.0.