Your question is not easy: as another OSX user I have the same concerns. Connecting to the internet is a risk--I evaluate it by figuring out what I have to lose and trusting that I've nothing very compromising on my hard drive. Really, my stuff isn't that interesting.

Easiest and safest way is not to share anything popular, leave the firewall fully on, and don't download anything that looks like bait. This means living as a leech. Your identity (IP address) will be available if you connect to the internet. I don't know how (or if it's worth the hassle) to learn how to disguise your IP.

Still, I'd rather take the low risk of a virus or hacker than be a leech. The whole idea of Gnutella falls apart if no one risks sharing. I've read of some people who only share sets of files for a week and then share others by setting up a series of folders. In the six months I've been trying to fully share (I want my upload stats to be higher than downloads) I haven't been hurt yet.

This is far too general. I'm interested in a better answer.