Hmmm drawing realistic people is a tough thing to do (as you know :p) I guess you just have to keep praticing with it...adding small details as you go, such as wrinkles, veins, etc. I found a tutorial from Google that might help you get started then work from there (Seeing as how I'm not great at drawing realistic or explaning things :P)
http://www.wikihow.com/Draw-Realistic-People
Observe people around you, look at photographs, or look at other people's art work. That's the best way to teach yourself how to do something 
Like I said too, keep practicing with drawing realistically. Comes with time D: