I can save you a lot of time and frustration in deciding which of these two major standardized tests to take and when by just saying this: Take them both twice with writing, and don't take them too late.
Let me explain that first part, the reasoning behind taking them both. The SAT and ACT are essentially the same test in terms of mathematics and critical reading, and anyone who attempts to explain a very miniscule or complex difference between the two is just making stuff up. The biggest difference is that the ACT also has a science section, and the entire test in general requires you to work a little faster than the SAT. For some unexplained reason, people tend to do slightly better on one test than the other, but it's really weird and random as to which test that is, so take them both - and make sure you take them with writing.
Currently, almost all colleges require them with writing, but the (potentially) good news is that your writing score doesn't actually matter that much. This is subject to change, of course, but as of right now and next year and probably the year after, colleges don't really care that much about your essays. Many schools don't yet trust the essay scores to be an accurate representation of students' capabilities because they are a relatively new addition to the test, so they mostly look at your SAT score on the 1600 scale: your critical reading score plus your math score. This sucks for people like me, who can rock an essay question but only manage a 670 on math, but it's awesome news for those of you who haven't yet mastered the art of timed writing and those of you who get nervous or bored or cynical when asked to try.
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Now the second part of that advice is the real trick. Don't take them too late. There is no rigid timeline you should be following. Collegeboard will suggest that you take them once in junior year and maybe twice your senior year, but unfortunately it isn't that universal. You should take the SAT/ACT your first time right after you take Algebra II or the equivalent. If you take it too early, you may be scared away by the advanced stuff and make yourself less confident. If you take it too late (in pre-calc is fine, Calculus is probably too late), you've already forgotten a lot of the basics or will overthink the questions.
The first time you take either test (ideally sometime between summer before junior year and summer before senior year), don't worry, don't study, don't stress yourself out. Tell yourself that you have another chance if you aren't happy with these scores, but try your absolute best, and you’ll find out what your raw score is.
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The second time you take either standardized test (ideally the summer before or fall of senior year), study some. Pick up a prep book, review some vocabulary, but again, don't stress. If you need to, you can take the test a third time in the winter or early spring of your senior year and send the updated scores to your colleges. Don't take either test more than three times, as there are rumors that some schools then begin to average your scores.
In terms of personal advice, I think you have to psych yourself up for it. You have to be in the mindset that this test doesn't define you; this test is an opportunity for you to showcase just how brilliant you really are. As an example, a lot of students think that in order to get a perfect score on the essay you have to quote someone historically famous or cite a literary work of significance or use huge, awkward words in all the right places.
I scored a perfect 800 twice - my first essay was about how the Internet is like The Breakfast Club in terms of bringing diverse people together (I had just watched it at a friend's house the night before), and the second was about my high school's scholar bowl team, and I didn't write a conclusion for either of them (in fact, I didn't even punctuate the end of that first essay because it was kind of in the middle of a sentence).
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You can't beat yourself up over things like that. Some prompts will be terrible or difficult or so open-ended that you don’t even know where to start; most of them probably will, actually, but remember that 99% of the people are thinking the same thing you are (why me, this is just my luck, can I go back to bed now, etc. etc.) and millions more have already been through that and survived. I promise, promise, promise, that you will, too, and if you let yourself relax and just do the best you can, you'll be surprised how well you do.
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