Get the lay of the land. Yes, you've already been in school for seven years now, but it is always good to know where you have been and where you are going. Look at the math you were supposed to learn last year and the year before. And look ahead at what you will be learning next year.
Talk to your teacher and parents and share with them how you really want to excel this year in math, and ask him to help you.
Ask your teacher for a diagnostic test to see how you rate with other eighth graders. Sometimes you can input your answers right online and get your grade for the test online too.
Take your test from last year and see how you did. This will tell you the areas you need to work on. Take good notes and know your weak areas. Start working on the eighth grade TAKS test a little bit at a time and get more and more comfortable with it throughout the year, getting and more right answers each time you do it.
Set up a study time every day after school for at least half an hour. This is your workout time. It's like getting in shape for track or practicing the piano; if you want to be good at something, you need to put in the effort.
Ask your parents to find you a tutor if you are having problems on your own. When you get stuck and can't find the answer it helps to have someone there that understands how to work the problem.
Keep testing yourself throughout the year. Hone in on specifics you're supposed to know at your age and master the material. And you will be ready to pass the eighth grade math TAKS test by the end of the year.