So just a note to start, you only really need this much detail if you're doing a headshot. If you are doing a full body, then the eyes will be really small and won't need as much detail. But for now, onto the tutorial!
Step one: The sketch! It doesn't really need to be perfect, you just need to map out the basic shape of the eye, the iris, the pupil and if you want, the eyelid, just so you know what you're doing.
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Step two: You just want to put a new layer over it and clean it up a bit, with a darker colour.
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Step three: Ignore that random line lol. Next, lay down your flats. The whites of the eye are never pure white, I used a very light beige to go with the orange in this one.
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Step four: Here is where it might get a bit tricky. It helps to have a grasp of colour theory. What you want to do is create the illusion of depth within the eye, by using colourful shadow and light. Here is a quick diagram I made:
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