This tutorial will show you how to retouch a dark photo to make it look brighter and more attractive

After:

Step 1: Open your photo in Photoshop. Go to Image > Adjustments > Curves (Ctrl + M) to go to Curves dialog box. Set something like the picture below:

Step 2: Press Ctrl-B to open Color Balance

Step 3: Increase green of the leaves by pressing Ctrl-U and choose Green from the drop-down box

Now our photo looks much brighter

Step 4: Go to Image > Adjustments > Levels

Step 5: We will make her skin smoother. Go to Select > Color Range and click on any point on her skin to sample it

Step 6: Go to Filter > Blur > Surface Blur

And here is my final result, the picture looks brighter and more attractive:

I hope the technique showed here will help you in your future work. But please remember to use your own judgements when playing with color and brightness