As NewRN2008 said, listen to as many lung sounds as you can in clinical and get with an RT. They're an awesome resource. Shadow one for part of a clinical day if you can.
Also, know normal. That is, listen not only to your pts in clinical but also check the lung sounds on your family and friends. If you know what clear and healthy lungs sound like, then adventitious sounds will jump right out at you. If you think it's rhonchi, have the pt cough, see if that breaks up & clears the junky sounds (usually they'll hack something out too, have a tissue available or a specimen cup if needed, know what orders are outstanding)
When ascultating, start with the bases and work your way up. Those deep breaths can easily tire a pt out, esp if they're older.
There are a number of sites that have examples of lung sounds, here's a good one that also has heart sounds: