Everytime I start a drawing I always pake sure the photo I m going to use has an excellent resolution. The better the resolution, the better the drawing (I am not very good at making up details). With a good resolution I can translate that into my drawing bringing enough expresion to the face. I especially put a lot of attention to the eyes. I think that most of the expression on a face is in the eyes. So here are a few basic tips for beginners that I've learned through my experience that might help drawing eyes:
the lower eyelid border has actually 2 borders, 1 inside that touches the eye and one outside that holds the eyelashes
Although they seem to, eyelashes are not very well organized. they usually cross paths and overlap
Always look at the shiny parts of the eye that are reflections of the lights. This bright reflections usually give the eye the sense of volume and bring them to live
The white part of the eye is not atually completely white. It has some grey and darker values on it. Most likely it has no white at all
The iris is usually composed by a set of rays that go from the pupil out. And it has a darker edge ont he outside.
Finally the tear ducts have always some shine...a little dot or dots of white there will give the eye the watery feeling.