Dear Hanzala, you are good student. If you want to get A’s in all subjects, you need to manage your time. Try to give equal time to each subject and little bit of extra to those which you think you are weak at or don’t like.
Follow a strict study plan and practice more and more. Keep a good attitude on all things and all subjects. If you have an "I don't care about this class or this subject" type attitude, then teachers will be less willing to help you with problems or offer you extra credit.
If you don't understand, asks someone! Read you’re textbooks and notes, and then if that fails, seek help. Ask a teacher, friend, classmate, parent, sibling, etc.