I enhance my photos a lot. I do other things than this but here is the basics.
If you have a dark photo like this.

You can brighten it up and it will make your siggy look nicer.

First you want to adjust the curves. Go to Adjust>>Brightness and Contrast>>Curves

I normally pick the anchor point in the middle and slide it over to the left. Then I will sometimes click to create another anchor point at the top and bottom and slide it over. You sort of have to play around with this but it is really easy. You can see it will make your picture look a lot brighter.

After you do this your picture may look sort of dull and washed out. So you will want to adjust the contrast.
Adjust>>Brightness and Contrast>>Brightness/Contrast

Then just increase your contrast until you like the way it looks.
