Ok I did a really quick thing. These tutorials are hard work... o.o
Okay, so, this is just an introduction to some of the effects we can use. Bare in mind I have Photoshop 7, not PS-CS, so yours might very well have more functionality than mine and perhaps a different layout... I don\'t know.
Anyways, make a new image. It\'s gonna be cloudy today, so use Filters > Clouds with your foreground/background colours set to white/black respectively. The thing with this filter is, it uses the colours you currently have selected; you don\'t get to select them in some \"filter preset\" window or anything. So keep that in mind.
Okies, I\'m gonna make these more stormy by going to Image > Adjustments > Auto Levels. That brings them out a bit eh? But it just needs a bit more... so, Image > Adjustments > Brightness/Contrast and drag the Contrast slider into the +70\'s. That\'s better.
Image 1There\'s what we have right now. A bit boring on it\'s own eh? Make a new layer and use the Flood Fill tool with any colour you like. Now I\'m gonna use a special brush shape by choosing it from the list of brush tips in the little box in the top left. It looks like some stems of grass... cool I guess. Anyways, we can make it alot more interesting than just a funky shape; look at these two pics.
Image 2Image 3These are showing you my settings in the Brush Presets window. You should find that as a tab in the top right (if not, it can\'t be that hard to find). Shape Dynamics controls the shape/size/angle jitter (or randomness) of the brush strokes. So basically, random = more interesting.
The second pic shows Colour Dynamics, which can make all the difference in a picture. I\'ve got the jitter slider all the way to the max for Foreground/Background, so the colours chosen as I\'m painting will be in the range between my currently selected foreground, and my currently selected background. And the colours I have right now are quite different... light blue and cream... this is gonna be interesting.

Right then, wave the brush all over the place a few times, and we end up with a pretty...... thing. Right now I\'m thinking, \"multicolour = good, shapes = not so\"... so I\'m gonna try and smooth it out a bit. Use a weak Gaussian Blur filter, an Auto Levels, and some repeats of the Gaussian Blur (you can repeat the last filter+settings you used by pressing the shortcut key Ctrl+F). Now I\'ve got this:
Image 3And now comes experimentation with layer effects. We\'ve done all this on a layer above the stormclouds, right? Well now we can combine them in interesting ways. In the layers window there\'s a little dropdown box that says \"Normal\" at first. Normal is boring!

Here\'s what you get by changing the multicolour layer\'s blending to Overlay.
Multicolour clouds
Or what if we change the order of the layers, place the clouds above the colours, and try something else... hmm... ah, here\'s a good example, Multiply.
Clouds MultipliedThis is cool because you can have a layer of line work, or shadows, and use it Multiplied above layers of colour to kinda blend it down into the colour, erasing any light bits of colour from the Multiplied layer and instead making all the colour underneath seen instead. I use this alot in my artwork to preserve outlines.
Note on changing the layer order: At first you might think \"wtf? I can\'t change the order, this storm layer is stuck\". That\'s because it\'s the default Background layer, and as such, is \"locked\" from doing many things a normal layer can do. To resolve this, drag the layer onto the little \"new layer\" button and it will be duplicated. Then simply delete the locked layer; you don\'t need it anyway.
That\'s about it... sorry for the quality, they are gif images.

Hope it teaches some useful basics.
(Hah, I fixed the brokedness of it all... hurray!)