Well there are a few ways to do this. The one I suggest is a none destructive edit, meaning that it does not alter the actual picture, so If you want to change it later you totally can.
Heres a screen shot I made
http://i21.photobucket.com/albums/b300/dyslexiaart/adjust.jpg
What you need to do is select the layer you want to edit then click adjustment layer, a menu will come up with a bunch of different adjustments (contrast, levels, hue, saturation) Choose the one you want. a layer will appear above the one you had selected. That is you adjustment layer. By clicking on diagram looking thing you can always change the adjustments. Next to it is a white box. that is your layer mask. If you paint in this area you can block out certain parts of the picture. Painting black lets none of the adjustments show through and white letting all the adjustment show through. You can see that I painted the rose area black, and left the background white. So my picture is the opposite of what you want. I made the background darker and left the rose the same. So if you paint everything black (in the adjustment layer) but the rose, your effects will only affect the rose.
Send me a message if you have any more questions
