Have you talked to the teacher at all about your performance? They person who can help you the most is the teacher. (Don't be afraid to, its their job to help you get through the class if you need help!) I personally am not a good test taker either.
But, do you study for your tests? If so, how early in advance. Try to start reviewing material at
least a day in advance.
Also, doing the homework (and by doing I mean - DOING!) will make everything so much easier. I know homework can get annoying and frustrating, but teachers don't give you homework to make you miserable, they give it to you because its what will help you grasp the concepts the best.
Like someone said before, practice exams are helpful - but, dont rely on these for everything you will need to know for the actual exam because the teacher may mix things up a bit.
Don't get discouraged! I am sure you can work things out if you try!
Good luck!