Thanks for your question. The answer really depends on how much math you've learned; both the SAT and ACT cover topics from Algebra 2, so you might want to have completed that class before taking a stab at one of these tests.
I tutor kids in NYC, and many of my students aim to take the SAT for the first time in August before junior year. Yes, before junior year! The idea is to prep over the summer and get the test out of the way as soon as possible. That way, you can focus on improving your junior year grades and excelling at your extra curricular activities, both of which colleges will likely care more about than your SAT scores, especially now in the time of COVID when many colleges are test optional or test blind.
In terms of prep, the tests are similar, but I recommend starting with the SAT and focusing your energy there. Everything that's on the SAT is also covered on the ACT (grammar, reading comp, math, essay), but the ACT tests more stuff (including harder math, and a Science test). So if you decide later on that you want to take the ACT, you can learn the additional topics when you decide to make that shift. To help make that decision, it's a good idea to take a practice test for each of the exams. You can find practice SATs on Khan Academy (all free, of course) and ACT on the act.org website.
Keep in mind that you can take the SAT as many times as you want, and that it's offered 7 times per year. Almost every student I've ever worked with has taken it more than once, so don't feel bad/dumb if you need a retake; it's very, very common and normal. Just keep in mind that you will have other things to do besides the SAT. Junior year grades. AP exams. SAT subject tests, etc. You want to get as many things out of the way as quickly as possible, so you can focus more on the other things.
Best of luck to you!