Your goal for these tests is to do the best you possibly can. Period. Find some study material, spend some time working on it, and then test. Do some physical activity the day before to help burn off the stress. Get a good night's sleep the night before, and eat breakfast the day of the test. You are not likely to suddenly acquire a new body of knowledge right before the test. However, you can learn test-taking strategy.
If it has not changed, the SAT penalizes guessing by taking off a fraction of a point for wrong answers. The ACT does not. You want to double-check this, as it has been many years since I tested. If true, this makes the ACT easier to achieve a higher score. If the schools you are applying to will accept the ACT, that is the one you want to take.
There is an art to guessing. I walk into a test with a letter in my head- usually B or C. For EVERY question that I flat out do not know the answer to, I put B. Out of ten questions, 2 - 3 will be correct!! If I can narrow down the choices, if B is one of the options, I put B. If not, I use the letter to the farthest left. So, if the choices are A or D, I'll pick A. If it is C or D, I'll pick C. Having a system like this removes a LOT of the stress that builds up around questions you are not able to answer, and saves a lot of time! Speaking of time, watch the time! If you are getting close to running out of time, rapidly mark all remaining unanswered questions with a B. If you still have time, go back and start trying to read them and enter the correct answers.
I taught the above process to a family member who had tried, twice, to pass the military entrance exam. On the third attempt, he used the method explained above. His score increased so much that they thought he had sent someone else to test on his behalf! They made him take it over, and fingerprinted him! He still got a high score!!
When you start studying for these tests, if there are certain types of questions you are having trouble with, seek out help! You do want to do the best you can. Just don't worry yourself sick over these tests!!