First check to make sure everything in your stethoscope is working properly. Secondly try and listen to as many lungs as you can. Are you a student or a practicing nurse? If the the former then your task will be easier as you will be exposed to a lot of potential lungs to listen to.
if you pinch the hair by your ears and rub with your fingers, that is similar to the sound of crackles. Crackles generally cannot be cleared by coughing (usually fluid in the lung(s). Ronchi is is more like gunk in the lungs, its a more distinct sound of air passing through thicker secretions. Morning is a good time to hear some good ronchi, often ronchi can be cleared or at least improved by some good coughs. Wheeze is that tight high pitched sould, almost musical at times, usually r/t inflammation. I agree with other posters, listen to everyone you can get your hands on! You will get the ear for it quickly if you practice on anyone who will let you have at them with your stethoscope. Good luck!