You need to learn with seriousness and diligence. When studying in class, listen and note what the teacher explained. Note what is important because it might not be in the book and later, it will come out during the test or exam. When free time, re-read the notes that were made earlier and memorize them while being understood.
If we feel good about a lesson, then test yourself with questions. After the question is done, check the answer with the answer key. Review the wrong questions answered again.