I wish I had known that I really didn't know anything.
Here is my advice:
Ask questions, all the time. Do not be shy, and never worry about looking stupid. Start interpreting labs as soon as possible. Get familiar with how the labs effect one another, and watch for subtle changes. If you don't know why someone is ordering a lab, ask.
Become really good at EKGs and radiology films. Look at pictures online, ask doctors and other practitioners to explain what they are looking at and how it is different from the normal. It is okay to admit you don't know what they are talking about. Make them clarify.
Listen to heart and lung sounds ALL the time. Ask providers to pull you in to listen to different sounds.
Buy a good stethoscope that works well for you, and learn how to use it.