Determine what areas you are struggling with. The subject of English is comprised of many different areas, including writing, reading, grammar and prose. Pinpoint why it is that you are struggling with the given area. Perhaps you don't understand how grammar works or maybe your decoding skills are lacking.
Discuss with your teacher ways in which you can improve the area in which you are struggling.. Ask if she can provide extra help after school or if she can suggest a way to better understand the problem area.
Devote time to studying the subject. Study without distractions and focus on the areas with which you struggle. Form a study group for added support. Avoid cramming in the days just before your English tests and instead, study for an hour or two each day. While cramming may improve your tests grades, overall, you likely won't retain the information you have studied beyond the test.
Seek the help of an English tutor. Talk to your teacher to see if he can recommend a tutor, seek tutoring information in your library or contact a professional tutoring company. One-to-one instruction from a professional can help increase your comprehension of the subject and improve your test grades.