The first thing I should tell you is that you can do it! As long as you put in the work, your grade is guaranteed to go up. I believe in you :)

1) Don't be discouraged.

Everybody fails once. Take it from a person who has had a 4.0 GPA their whole life, because I've had my share of failed quizzes. If you've been working hard and you still get a bad grade, that should be your motivation to work harder. You might not see an improvement right away, but that's no reason to stop. Keep working, and it's impossible NOT to get better!

2) Avoid distractions.

When you're studying or doing homework, turn off your phone. I'm serious. If you're an iOS user, hold down the power button and actually turn off your phone. If you need to look something up, use a computer. This might seem extreme, but it's really important that you are undistracted while you are doing work. Turning off your phone and putting it in a place where you won't see it (out of sight, out of mind) will help you get focused and remember information better.

3) Get better sleep.

Notice how I said better, and not more. (Well, more sleep is better, but...) If you go to bed stressed, you wake up stressed. Before bed, try to meditate a little (there are a lot of useful apps for this. I recommend Headspace) or just take a few deep breaths and forget about anything that might make you anxious. Try to avoid sleeping with your phone nearby, because that will tempt you and make it harder to get to sleep. Anything you can do to develop a bed-time routine (for example, I always read before going to bed) will help also. If you're one of those people who sleep at 2 AM every day and it's hard for you to go to bed earlier, try to take small steps, like going to bed 5 minutes earlier every day. Finally, to maintain the proper circadian rhythm, DON'T sleep in on the weekends (at least not extremely). Doing this messes up your sleep cycle and will actually make you more tired on the weekdays. I know this is super hard (especially for teenagers) but if you can wake up on the weekends at around the same time (give or take an hour or so) that you do on the weekdays, you will see better results.

4) Take time every day.

Choose a class. It could be your hardest class, or the one that has a quiz coming up. EVERY DAY, take around 10 minutes or more to review what happened in that class. Pretend you're the teacher, and teach yourself. This will guarantee the information is stuck in your head. If it's a math class, or one that involves solving something, make sure to do a couple of practice problems.

5) Take advantage of online resources.

Khan Academy is a GREAT website to review certain areas (mostly math) On the weekends, if you have some spare time, go on Khan Academy and just do a couple of problems or watch some videos.

There are also a lot of videos on YouTube that help with a variety of subjects. Crash Course covers so many things, including Chemistry, AP U.S. History, and more. Again, in your spare time, try to look them up and watch a couple of videos, even if it's only review.

6) Always ask questions.

If you don't understand something, ALWAYS ask someone about it. Don't worry about what they think, the most important person is you. The best person to ask is your teacher, but if you feel really uncomfortable doing that, your friend or your parent might be a good second option. Whatever you do, make sure you understand the concept you have a question about before you move on in the class.

7) General tips:
Do your homework.
Pay attention in class.
Study. (obviously)
Come prepared.
Be organized.
Participate in class (this is a great way to get easy points).
Avoid being absent.
Form a study group, if you think it will help you learn better.

That's it! I hope this was helpful! If you have a personal trick that helps you study or get better grades, make sure to comment it so other people can see!