So, the left is what we start with and the right is what we're gonna end up with. If you want, you can download my original image here and follow these steps to practice.
I always start by making a curves layer. I always have the Adjustments-panel right there, but you can also create a curves layer by going to Layer > New Adjustment Layer > Curves. We're gonna lighten the photo up a bit by dragging the curve up in the middle. You should play with this a bit to see what works best for the photo you're editing. I make sure I don't overexpose my photo's when I take them, cause it looks better to make a darkish photo lighter in Photoshop, than to make a light photo darker.
Now we're gonna make a gradient layer. I always make mine in pink tones for a summery effect or blueish for a colder effect. Again, play with this a bit. Of course we don't want our photo to be all pink, so we're gonna do the following:
We're gonna set the layer style to overlay. This is gonna create a hideous '90s effect, but don't worry, we're gonna tone it down a bit by setting the opacity of the layer to somewhere around 25-30%, and then it'll look like this:
Almost there! Now we're gonna select the part where the light is coming from. In this picture it's the upper left side. Select that part and feather it (Select > Modify > Feather).
Then we'll turn that selection into another one of those lovely pink gradient layers. You'll do the same as before, but not as an Overlay, you set the opacity of the layer to whatever you think looks nice and natural. If all goes well, you'll end up with something like this:
Aaand you're done!