You can make a big improvement to photos which are too dark by doing a couple of things in photoshop.
Open the photo in photoshop.
Type Ctrl+L (hold down the Ctrl key and press the letter l on the keyboard). A small window opens:
To adjust the input levels using the graph, drag the white slider to the left, until the photo looks how you want – you should probably stop just to the right of the “1.0”. (Make sure the Preview box is ticked so that you see the effect of the changes.) Click OK.
You can also then click on Enhance, then Adjust Lighting, then Shadows Highlights. A window opens, and you can play around with the sliders until you like the result (again, make sure the Preview box is ticked so that you see the effect of the changes.) Click OK.
If you are happy, save the photo.
If you have a photo where, for example, the subject in the foreground is too dark, but the rest of the photo is fine, you can make changes just to the dark part of the photo.
Click on the Marquee tool, and using either the rectangle or the ellipse, draw a marquee round the area of the photo you want to improve, and follow the steps above. The changes you make will only be applied to the area inside the marquee. When you are happy, kit the Esc key to get rid of the marquee, and save the photo.