Step 1: Making peoples eyes a crazy glowing colour...
So open your document and create a new layer, there's a little button just above the layer views. Set the blending mode to overlay, that's done using the little drop down menu that has the default normal, found just beside the new layer button above the list of layers.
Make sure your new layer is selected, click on it if you're not sure now take the brush out and make the brush size about the same of the eyes, select a nice bright colour and simply paint on over the iris (coloured part of their eye's).
That's it they should now have funny coloured eyes!
For more subtle/realistic effects choose a bright colour anyway but lower the opacity of the brush or overlaying layer.