There are SAT/ACT books available at your local bookstores. You can probably purchase one or two of those, and just do a general review of what is in them. That helped me, for the most part, but I'd also recommend studying your vocabulary for the SAT. The English portion of the exam is vocab heavy, so if you have any problems with English and sentence structures at all, I'd strongly recommend studying that. I never took the ACT, so I can't help you there, unfortunately.
But overall, with both tests, you should try your best on each section, and if one subject isn't your strong point, just do what you can and re-take it later if you feel that you can do better. If you take the tests multiple times, you just use your highest score on your college applications.
Just take a deep breath before each section, get PLENTY of sleep the night before, and bring a water bottle into the test with you. Sometimes you need it as a wake me up. Haha. I'm sure the test may seem frightening, but you -can- retake it. So don't sweat it, and just stay positive during it~
ALSO! Don't let anyone tell you that taking it more than once is stupid. I took it twice, and did wayyyyy better my second time around. xD The jitters/nerves of test taking always get me. Good luck! <3