The purpose of this tutorial is making an image more appealing. We'll work with
this image made by cdcguard and posted at
morguefile.
As the image is pretty big, we're gonna work with a reduced version:

My result after following the steps explained in the tutorial:

Sharpening the image
As you can see, the image is a bit blurry, so we're going to fix that.
1.- Duplicate your base layer (the layer where you have the image) by clicking
Ctrl + J or going to
Layer >> Duplicate layerThis is important. When you're going to modify an image, always work over a duplicated layer from the original image. In case you don't like the effect, you only have to delete this duplicate layer.
2.- Go to
Filter >> Sharpen >> Sharpen to sharpen the image.

Making the image look realistic
Our image is now a bit dull. To fix that, we could use some Adjustment Layers, but let's try something different.
3.- Duplicate your sharpened layer, and set its blending mode to
Soft light

If your original image is too dark, you can use some
Screen layers too.
Coloring
As you can see, the image colors look a bit weird, maybe too blue. So now, time to some color effects.
4.- Create a new layer
Layer >> New >> Layer, and using the
Paint Bucket tool (

or pressing
G), fill this layer using a dark blue #040e35.

Set this layer to
Difference

It still needs some more color, so...
5.- Create a new layer again, fill it with the same dark blue, set its blending mode to
Difference and its opacity to 50%.

And that's it!
Final result

Layers used:

I hope you liked this tutorial. If you have any questions, or something isn't clearly explained, please tell me.