At the university level, it is assumed that you will spend three hours outside class studying and doing homework for every hour you spend in class. That means that if you are taking a full load of four classes, you should plan to spend 30 to 50 hours per week on studying and homework. In other words, going to university is like having a full time job in terms of the amount of time and effort it requires to do well.
The first strategy you should use to study effectively is to minimize distractions. That means turning your phone, computer, television, and other electronic devices off completely as you study. Although quiet instrumental music may help some people concentrate, any songs with words may interfere with your studying. If you do not have a quiet space at home or in your dormitory, consider reserving space in the library.
While some people can study efficiently in groups, many study groups degenerate into social hours rather than actual study time unless all the members are very focused.
For paying attention in class, studies have shown that students retain more information if they take notes by hand than if they use computers. Turning your phone off will also help you. Try to sit near the front of the classroom with the serious students to avoid being distracted by the students who are whispering and checking their phones rather than paying attention.
Finally, healthy habits, including getting a full night's sleep, eating a well-balanced breakfast, and getting some exercise (even just a few 15-minute walks) every day is correlated with improved academic performance.