If you want to understand some principles (not implementation-specific and not specific to QGC) of MAVLink, here's a short guide, definitely worth a read.
However, without more information like
* which camera are you using? (Is it a FPV camera, does it support CHDK etc.)
* How did you connect the camera to the Pixhawk? (AUX Port?)
* What do you actually want to achieve?
so do you wanna turn it on? Or make the camera take a picture (assuming the camera is turned on manually before flight)?
Note that QGC comes with quite some camera support out of the box, so maybe there's no reason to re-invent the wheel. Maybe you run in this issue though?
Also have a look at the MAVLink message definitions, they give you a great overview of what the protocol is capable of per default, even though the information might overwhelm you, but
Ctrl+F is your friend.
Maybe you just want a button on your remote to trigger the shutter of your camera? As I said, understanding your use case might help a lot