The first photo is the original, with color editing already done, however the flowers on the left are a little dark for my taste so I wanted to lighten them just a bit. This effect is subtle, as you can see by the final photo (on the bottom). From previous posts you should know how to create another layer so I will skip that here. The new blank layer is going to be a gradient layer of white and transparency (in the gradient tab you'll choose FG to Transparency with white selected as your foreground color, but you could be artistic and use a sepia or another really light pastel to create a whole new effect, but for shadow correcting, you want white #FFFFFF).
The second photo above shows just the gradient on how it was placed on the photo (black appears only in this tutorial), using "linear" and sweeping in the direction of the light from left to right. The third photo shows how the gradient looks over my original photo. Change your layer mode to "overlay" for the gradient layer. If it's not bright enough for you, duplicate your white layer and adjust the opacity slider as needed.
**You can create shadows with the same method but use black #000000 instead of white. You can create starburst or pinpoint lighting by using the settings for the gradient style as radiant instead of linear and FG/BG colors of black and white (no transparency). To make your radial larger, start your gradient off the photo. Play around with it to get your desired effect but keep in mind to "undo" your gradient if you are not satisfied the first time because in transparency mode the gradients will "pile up" on that single layer.
Once you have mastered this technique, you'll find it to be a great way to lighten up faces that are too dark when the photo was taken without a flash and the light source is behind them. Just a tip - don't be afraid to use flash in broad daylight, even at the beach. In an auto setting it won't come on in these conditions so you'll have to change your settings or depending on your camera, force a flash.