This is easier than you think! You do have to work at it, though. It doesn't matter what your normal grades are (unless you're already an A student) you can improve through these simple steps.
Take notes on everything. For most courses in high school and junior/community college, most of what is on the tests and homework will be discussed in the class. If your teacher draws a diagram on the board, copy that down - it can help you remember the information. Don't just copy what they write on the board, either: take extensive notes on everything
Assignments comes first over everything else: set yourself goals and make sure you reach them before you go out and party. Social life is important too, but make sure your work comes first. But don't become a social outcast because that is as unhealthy as bad grades. Keep the grades up and have a social life. When you get a bad grade just think that you can still bring it up in the future, so don't worry. Try to do your best and most importantly ask a teacher for help if you need it!
With assignments, don't travel into unknown territory, use your notes to answer what the teacher is asking for: if you see something that you're not sure is true, or that you're not sure your teacher will agree with, don't say it. Do it correctly, checking to make sure you have no mistakes, and you can't mess up. Look over your asssignment in detail at least once before you turn it in, and if you're uncertain about something ask your teacher!
When taking quizes, relax. If you're fidgety, or worried that you'll fail the quiz, you're sure to get a low score. If you studied for it long enough that you know the information, then you shouldn't be worried about getting a low grade. Remember the most important advice to getting good grades is very simple; do well on quizzes to get an A or an A-. Do your assignment and do what your teacher says, and the chances are fairly good that you'll get an A+. Don't stress yourself. Act cool and calm.
Watch to determine how your teachers teach, use the same method to learn the information. If they write on the blackboard, take notes; if they lecture, listen hard or record the lecture to listen to multiple times. This sounds simple, but many will find it hard to stay focused for an entire class period. Here is where the hard work comes in - staying focused.
Time management & Organizing is essential. If you're given an assignment that's due next week, don't procrastinate; get started the same day you get the assignment
A lot of people say this but it really does help. Organizing your locker helps you remember to bring your supplies to class and to your house to do your homework or studying. Organizing your notes will make them much easier to understand later on. Also keep your desk/study space tidy, and make sure you have a clean, quiet place to study and do homework that is withdrawn from the main part of the house and from noise.
Get good sleep. You need sleep in order to stay focused and with out it you will have a hard time staying on task and you will forget what you just learned . So get lots of sleep and always eat a good breakfast. The breakfast will help you stay focused along with the sleep.
Don't study all in one night, study a bit everyday. Your brain cannot take in so much information all at once. Make points, have a friend over, or join an after-class group discussion if you can for help from a teacher or a friend from the same class. A lot of people think studying is "lame" or "boring". You can make up a board game to make studying fun and easy, or you can just type up your notes if it makes it easier to read. Do as much as possible to make sure you know the material.
Eat a good healthy breakfast . it is known that if students eat a good and healthy breakfast every morning they are known to get good grades in institutes and are able to focus better.
Have good relationships with the teachers. If you don't they may give you an A- instead of an A if you are mean, don't pay attention, or don't listen to their advice, it can affect your grade