You need to hit “Q” key. Then select the brush tool and make sure that you have the foreground color set to “Black”. You might also want to set your brush to about 75% opacity.
Go to your picture and just “paint on” the quick mask. Once done hit the “Q” key again to get out of quick mask.
This will select everything BUT what you wanted. Now hit “Ctrl+i” to only select what you painted. Then hit something like curves. Photoshop will make a mask for you on that layer.