How do I adjust contrast on an image in Photoshop?

To balance the dark and light (black/Shadows, white/Highlights) values on an image in Photoshop:

Duplicate the image layer and select it. 

On your right-side panel, select Adjustments, then Curves (you may need to select this from the bottom row of New Adjustment Layer icons - "Curves" is the black/white circle icon).  The eye droppers going down the left side from top to bottom will allow you to adjust the Shadows, Midtones, and Highlights.

Try to find an area of the image that is supposed to be white (the brightest area) or black (the darkest area).  For this example, I am going to use the black on her dress for the darkest area.  Select the top eyedropper for black/Shadows, then click on the black area of the image.  Here is a before and after:

Here is another one where I selected the bottom eyedropper for white/Highlights, then selected a white area on the image file.

Then I went in again and did the black.

And finally, I went in and selected a different white area that was a bit darker than the first one: 

If your page is not set up like mine, you can find the eyedroppers by either clicking on Layer at the top of the screen, and then New Adjustment Layer and Curves,  or by creating a new fill or adjustment layer

(  ) and then selecting Curves.