An exception to this is if you are planning to declare a major in math, science, or engineering, you want to make sure you can submit a score with a strong math section. For example, you might be better at the ACT overall but do better on the SAT math portion. In that case, it might make sense to try to work on and submit both tests.
As a note, schools can sometimes be concerned about getting high standardized test scores from students with low GPAs. They worry that might indicate a student who is intellectually talented but academically lazy or struggles with the mechanics of keeping up in school. So make sure that your essays, supplemental material, and recommendations explain the disparity between your GPA and your test score. You want to note if your GPA has improved or if there are extenuating circumstances like a long absence from school or an illness that made it harder for you to get good grades. You should explain this in your essay, and your guidance counselor or teachers need to corroborate it in their recommendations. You want to help the colleges feel confident that your high score represents strong natural ability that will translate into a better GPA in college than it has in high school.
Although I would double-check which ones do. Due to Covid a lot of schools are no longer taking scores just GPA but if your GPA is not as high I would just take it to strengthen your application and to show you are a good student.
There are a lot of different resources such as Khan Academy that you could use to get a feel of what is coming or what you may like. I also believe you can sign up for class. I signed up for SAT classes and they taught me how to read the book and answer. Of course, they don't give you answers but it is good practice if that is what you are looking for.
If you don’t have time to retake them, you could consider getting one of those practice books and studying a bit beforehand so you know what to expect in test day.
Good luck! You got this :)