Step 1: Pre-read, Literally never understood the point of this or knew how to do it. I felt like if I pre-read I would just be studying and not know what the hell is going on. I didn't figure out how to use this tool effectively until later in medschool. It supposed to give you a general understanding of what youre supposed to learn. You could realistically do this 10mins before class. Thats all it should take really. 10 mins. "Oh we're doing photosynthesis today - so he's going to tell me how plants make energy". Thats the type of knowledge level you want to pick out with pre-reading. Not big things like transport mechanisms or enzyme names, just your basic fundamental "What is the point of this lecture"
Step 2 (what you mentioned): If it sounds important to you...make a note. It's YOUR knowledge, your brain will pick up the story more easily if you pay attention to what signals your brain is giving you. I'm guessing you have hand outs or .ppt's that the prof will give you. Whenever he says something you go "oh shit...i better know this" Just write it down "This enzyme is for this transport". 5-10 words max for notes. Don't get bogged down with sentences. Remember, YOU wrote YOUR idea. Youre going to remember YOUR story in shorthand later that night (or even the next day - not recommended and I'll get to why later).
Step 3: Review those notes: Post Read - Literally 10 mins. Glance over the notes and make sure you know what was talked about where in the lecture. "Hey Dr. Nye defined Photosynthesis here, he talked about how it occurs here, he mentioned the storage cells here, and he listed off a bunch of them here". This is reinforcement.
Step 4: Later that night, re-write your notes in your own words and create your own "transcript". It won't be word for word from the professor. It's going to be word for word from your head. What you remember. If there is something you forgot - hey look at that, youre learning - you have figured out what you need to spend time on. You'll look it up, you'll see where this forgotten knowledge fits in your brain and you'll place it accurately. This usually ended up talking me 30mins for short lectures, 1-2 hours for longer ones.
Step 4: Review these notes that you created for yourself. You don't have reread a 30,000 word section on photosynthesis since you've created a manuscript of 1000 words (of your OWN story). This took me 30-45mins.
Step 5: Relax. You've put in the work.
Going back to why you want to rewrite those notes w/in the same day - because it's proven that memory retention dwindles rapidly if you dont review the same day. Sometimes you can't help it, but it's VERY important.
I list this out like this because it's important to remember that everyone is different. What one person struggles with, won't really be someone elses challenge. Thats why the way to improve in anything is to improve YOUR weaknesses. Seriously, I'm not smart. I just work my balls off...and had I figured out these things before medschool, yes I would have still had to work hard, but I would have done so with far less stress.
If I knew this in college...I would have been able to party literally every night with only a minimal drop in GPA.
Everyone is different, but this way worked for me (extremely well...).
Oh and just re-reading your statement: How do you pick whats important? Well focusing on your lecture this way will eventually give you all the answers. The best way to guarantee an A is to know everything. You may not study everything, but you'll be familiar enough to start making guesses correctly.