Consider also talking to local tutors who can work with you in person and help identify specific areas that need improvement. All the best to you in achieving your academic goals, and thank you for asking!
High School Success and College Application is a book that will likely help you in studying as well as many other areas of high school. Grades are important, and it's also important to take a big picture view as you go through the rest of high school.
In the book, the author explains key points and fundamental strategies regarding class selection, grade point average, test skills, attitude, extracurricular activities, honors, awards, recommendation letters, essays, interviews, and college applications.
Create a positive attitude. You may ask "how does creating a good attitude supposed to boost up my mark?" Well, if you create a good attitude in studying and think positive things about studying, it's likely that you will start liking a subject that you were really not fond of before. You may also get marks for having a good attitude and for not disrupting a class.