If you have some familiarity with the topic it will be easier to follow the lecture and regognise the important points. In a lecture where the information is new and unfamiliar to you, it may be difficult to know what to write down - you may be worried you will miss something important. Paradoxically, the more you write, the greater the danger of missing something important. Note taking should not be a speed-writing exercise. Try to listen for the main points and get them down clearly.
Lecturers often give clues which indicate the most important points.
You may be researching for an assignment, or preparing for a tutorial, or an exam. Keeping this purpose in mind will help focus your reading. Concentrate on the introduction and conclusion, and the opening sentence of each paragraph, and note the main points. Extract the needed information. Note the most relevant sections and summarise them as necessary for your purpose.