It might help to first identify the reason for your poor grades. How do you learn best?
For example, I learn through writing down everything the teacher says. I then go home and re-write all of my notes. This repetition helps me to better absorb the information. If I'm having a lot of trouble I'll generate my own questions based on the material, or find study questions in my text and make flash cards from them.
Try thinking of real world applications of your material, or catchy phrases to help you remember things.
Developing good study habits is the first step to higher grades, but sometimes the subject is still giving you issues. At this point you should look into a peer study group, or an after school tutor.
Finally, Get lots of sleep. A lot of students make the mistake (myself included) of cramming until 3am for an 8am test. This won't help you at all. Make sure you get a good eight hours of sleep before any test, and be sure to eat a healthy breakfast every morning.