First you need a decent picture. Bigger pictures work best, because you can always make them smaller and trying to make one bigger ruins it. Also make sure the picture is of high resolution. To practice, just use this:

Once you place the picture into Photoshop make sure you flatten the image. Then duplicate the layer by going to Windows>Layers. Right click on the image and duplicate it. Your layers box should look like this:

Make sure the duplicated layer is on top, and selected. Then take the dodge tool and set Exposure to 75% and the Range to Midtones. I'm using a size 28 brush with soft edges. Then starting at the iris going outwards color the whole eye, it should look like this when done:

Next pick the Eraser and set the Mode to Brush, Opacity 100%, Flow 100% and I'm using a size 35 soft edge brush. Erase the iris, this should only take 2-3 clicks, then on the forth hold down the mouse for 3-4 seconds, this make it sort of flutter out...It should look like this when you're done:

Alright, next we're going to pick a color, I'm going to make my eye purple. So go to Imgae>Adjustments>Variations, this is what you click:
More Blue, More Blue, Yellow, Magenta, Red, Darker It should look like this now...

Now, we don't want a red face do we so pick the Eraser and set it to a soft edge brush, size 65 and erase all the coloring except for what's on the eye. BE CAREFULL ERASING EDGES, you might want to drop the brush size down to get the edges of the eye good. Should look like this once your done:

Now, take the Burn tool and set it to size 9, soft edge, and set the Range to Midtones, Exposure 16% and trace the very edge of the eye. The switch back to the Dodge tool and go around the inside of the eye. This creates depth. It should look like this now:

Now, if you like this, you can leave it like it is. but I'm going for more color. So, in the image tab, go to Adjustments, Levels. In the Channel box select Blue, and set your settings to these: Input Levels: 0,0.69, 255. Then change the Blue to Green and match these settings: Input Levels: 0, 0.69, 2004. If you want a more blue color, Select Red and match these settings: Input Levels: 0, 0.6, 255. You best bet is to mess around till you find something you like.
You eye should look like this now:

Alright, now we need to do something with skin tone, in your layers box select the Background layer, the bottom one. Then go to Image>Adjustments>Hue/Saturation. Make sure the Edit is set to Master, and change the hue to +7 and the Saturation to -66 and leave the Lightness at 0. It should look like this now:

And you basically done! If you want to add more depth to the eye, take the Burn tool and darken the top of the eye, and use the Dodge tool on the bottom, overall just experiment.