Look over all of your assignments so you can see exactly why you have the bad grade. The grade could be a result of low test scores, missed assignments, bad grammar or wrong answers. It is important to understand why your grade is low before you can attempt to pull it up.
Contact the teacher or professor and schedule a short meeting. Tell him that you would like to improve your grade and ask what you can do to accomplish this. You might be able to do some work over or take some extra credit. Ask your teacher or professor for advice on what you can do.
Inquire about tutoring services that are usually offered by the school if the problem is lack of understanding or poor grammar. If your grade is low because you failed to turn in some work then see if this can be made up, even if you will be penalized with a grade reduction. Your grade will still come up if this work is completed.
Plan to start studying harder. Get organized and prioritize if you want to get better grades. Learning the materials, applying what you have learned, and turning the work in on time is how people get good grades. If you see yourself struggling it is up to you to ask for help.