What you need to do is as follows, I have CS2 mind you so not sure if it will be the same,
Simply go to ‘Image’ ‘Adjustments’ and click ‘auto levels’ or if you are feeling confident choose ‘levels’ and manually adjust them.
Then to reduce the noise (grain) you need this little program (neat Image):
http://www.neatimage.com/download.html
Its fairly easy to use, open the picture, use the default settings, it looks at your picture and reduces the noise and saves it under a different name.
Also always save your pictures under a new name, so if you mess up you have the original still.
(One more thing, if your camera has it (which I doubt) shoot in RAW mode as you can adjust the exposure and settings better afterwards)
Hope this helps!