It will warn you if you are at any risk of losing data.
If you previously sync'd it on a PC then yes you will lose everything that's on it. But again, you'll get a warning before that happens so just cancel if you get the warning, then let us know and we can probably guide you from there.
I'd be curious about that, as I'm contemplating switching to a Mac. I have my iTunes library on an external drive a the moment, but hadn't given any thought actually to the first sync. So if I hold down the Command key (or is it the Control key) when opening iTunes for the first time on the Mac and point it to my library on the external drive and open that library, it will still wipe out the files on my iPod Touch?