Streaming, how does it work?

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Hello all, I am a SCII player (obviously sorry), anyway, I was going to get into creating a tournament, and stream the games live etc.

Anyway, so I thought I knew how to do it, I created a twitch account and got setup, got ready to do a trial run, and realised that I don't have any software to stream from my SCII game to my twitch thingy :P I have a mac, does anyone know any good software for this, or am I missing something obvious, like the mechanics of twitch. Sorry, kind of new to this. 

Just FYI, I can see the game on my laptop, I want to transfer my laptop screen (in-game, as I am observing) to my twitch, and for live streaming! 

Thanks in advance :)