Try to put things into your own words when you can . This is not always possible or even a good thing because the lecturer's words may be more precise and meaning-laden than your own. However, when you can use your own words, do so because you are more likely to understand and remember your own way of seeing things. When you write down ideas you don't understand, get assistance after class or, if appropriate, ask at that point in the class. Listen for teacher cues that inform you that important material is coming.
Voice changes can also indicate that important material is coming.
A particular interest in a topic by the teacher should probably indicate to you that this is an area to learn well. Review your notes as soon as you can after the lecture . Fill in gaps, mark important points, put question marks beside things you don't understand, fix barely legible writing, and put summary points in the left hand margin. As you identify those areas of confusion, get help from the textbook, another book, a friend, or the teacher . You should be working towards reviewing your notes at least three times to maximize learning.