There are so many options for djaying and then posting the output online it's hard to know where to start, but as a suggestion (this is not my area of expertise), you might do a search for "dj" on the Mac App Store and MacUpdate.com and see what comes up.
You seem to be fairly serious about it so I'd point you to algoriddim's
website where they've just introduced djay 2 (if you used the old one you might want to take another look) and
Native Instruments which will let you get as serious as you want with their stuff (but its not a cheap hobby).
I've played with VirtualDJ Home because it was free (and worth every penny
), the Genius mix combined with crossfade in iTunes (for those of us who can do pitch and beat matching in our heads ...) and vjay for iPad (which is great fun). You can take the audio output of pretty much ANY audio-based program and put it online live through tools like InstaCast, NiceCast (recommended, works with Shoutcast 2), and there are several other options. Do a Google search.
