urm.. if anyone could explain the 'steps' from turning a child painter into a leonardo da vinci.. i think he'd be making quite a name right now.

anyway, there isn't a shortcut or an easy step-by-step tutorial to teach you how to turn a hand drawn drawing/sketch into something like the example you gave.

it is however possible to first learn the basics of Photoshop to create a 'dumbed-down' version then later progress on yourself via experimentation to find your own style and preference that would simulate the results you're looking for.

So my suggestion is:
Learn the Lasso and Pen tools well. Then discover gradients and finally, learn to control brushes.

that's my easy steps of getting what you'd want.

well, it's kinda made to resemble my face though i've got mix comments. hahaha..

i used photoshop and played with the brightness and contrast to get a rough idea what I wanted. then i used that same image in Illustrator to trace.