In this tutorial I will show you an easy way to get a cool mist effect, like the picture below

Firs thing you need to do is open a new file, I made mine 400x400 px. Then select a portion of the bottom right corner. Now make your for-ground color black, and your background color white (or just press "d") And then go to filter>render>clouds

Now use your smudge tool and smudge it outwards

Now create a new layer, and make the layer type "color" and fill it will a nice blue color. It should now look like this

Now merge all your layer. Now duplicate your layer that is left, and make the layer type "color-dodge" . Now merge the layers again, and ad some more smudge. Now you should have something like this

And your all done!