Step 1 We'll start off easy. Make a simple gradient background. This is ocean, so make it some shade of blue.
Step 2 Now draw the body of the fish on a new layer. My drawing tool of choice is the pen tool. If you're more comfortable with some other way, feel free to do it your way. Once my path was complete, I made it a selection and filled it with a solid blue color. (Don't worry, we'll shade it and add texture later)
Step 3 More drawing, this time it's the top fin. Everything I do will go on its own layer. I called this layer 'Top Fin'. Fill it with a yellow color.
Step 4 These are the flippers. I called the layer flippers. Fill it with a light grey.
Step 5 I called this one Bottom Fin. Also fill it with light grey.
Step 6 This is the tail layer. There will be another layer between the tail and the body because it is a different color. Fill this layer with light grey.
Step 7 This is a stripe just around the 'face' of this fish. I simply called it stripe layer. Fill it with light grey. It will not appear in the next few images because I was hiding the layer while drawing the next ones.
Step 8 This is the face layer. It should be black.
Step 9 This is the side fin layer. Fill it with yellow.
Step 10 Now we have the Tail Base layer. Fill it with yellow and move it below the Body layer.
Step 11 Draw a circle for the eye and fill it with a darker grey You should have something like this so far.
Step 12 I had to trim up the Body layer a bit, but now you should put a mouth in. I zoomed in real close and made some 'fishy lips'.
Step 13 Take out your trusty burn and dodge tools and burn/dodge the body and the top fin. Burn tool goes on bottom, dodge tool goes on top (because this is a fish -- the light source is obviously coming from the top)
Step 14 I used the smudge tool on the top of the fin to give it a little more fading effect. Also I went back over after smudging and dodged some more.
Step 15 Burn/dodge the tail and the tail base layers.
Step 16 Make a new layer above tail (tail border): ctrl-click the tail layer, fill tail border layer with black on the new layer, go to select>modify>contract enter 5, hit delete. This will make a 5 pixel border around the tail on a new layer.
Step 17 Use the smudge tool on the tail and tail border, set the opacity on the tail border to about 75% and the opacity on tail to about 50%.
Step 18 Now we're going to start making the texture for the tail. Take a 1px paintbrush and draw some black lines on a new layer above the tail. They don't have to be perfect, or even straight. Just in the same general direction. Call the layer Tail Texture.
Step 19 Duplicate the layer, and put a color overlay of white on the new layer. Select the move tool and push the up arrow once, and to the right once. This moves the new white layer 1 pixel up and 1 pixel right of the black layer.
Step 20 Press ctrl-e to merge the white layer down to the black layer. ctrl-click the tail layer to select it, and go to select>inverse to inverse the selection. With the Tail Texture layer selected, press delete to get rid of extra lines.
Step 21 Change the Tail Texture layer blending mode to soft light. Set the opacity to 35-40%. Move it under tail border layer, but over the tail layer itself.
Step 22 Do the same thing for the bottom fins and flippers. I won't take you through each of those steps, just follow the example of the tail, but omit the black border.
Step 23 On the side fin layer, use an inner shadow, and paintbrush/smudge tool at the base on a new layer.
Step 24 Add texture to the top fin just like we did on the tail and other fins. Burn/dodge the stripe behind the face, and smudge the face a bit. Create small white stripe under the eye on a new layer and smudge a bit to give it a little reflection.
Step 25 Time to work on the eye: Color overlay with dark brown, use an inner shadow and bevel. I moved it a bit closer to the edge because it didn't look quite right. On the mouth I created a new layer, drew a stripe down the middle to separate, changed the opacity, used blur tool, (on stripe) dodged and burned mouth @ close up. I smudged the stripe layer a bit more as well.
Step 26 Now we're going to create a texture for the body. Make a new 10X10 image and draw a 1 pixel X in it. Save this image.
Step 27 Now duplicate the body layer and use the glass filter. Load the texture you created (the file with the X). Set the blending mode to darken then use spherize. After using spherize you'll have to adjust the scale a bit. Do the same thing to the tail base That's it. I ended up merging the body layers and adjusting the color because I was a little unhappy with the blue. There are probably still some things that could be done to make it better, but for now my work is done. You should at least have some idea of how to get started drawing your own stuff in Photoshop.