When do I start preparing for the ACT and SAT, btw…I am a sophomore. 
Probably this summer. Try to take the tests during your junior year. This way, you don’t have to stress your senior year when you re applying for colleges. If your scores aren’t the way you like them your junior year you’d have plenty of time to retake them. Good Luck! 
In fact, you begin preparing for these tests in primary grades. These tests sample your reading, writing and math an Los developed over time. Your scores are made up of the number of items you got correct with the assumption that higher scores are associated with greater skill. Individual test items are pulled from a large pool of test items. But nobody says be sure to know King Lear, the Battle of Hastings, and so on.
Your goal is learning whatever you encounter n your school career. That us a separate issue from earning A grades! Waiting until these tests are approaching to learn lots of content is nuts. Instead, you need to be able to perform at a recognition level. If the question asks for the name of Henry 8ths wife, I don’t need to look at answer choices. Instead, I mark Catherine of Aragon and move to the next question. That is far easier than learning the names from a test prep company and that question isn’t likely to appear anyway.
So, learn information and skills as you go through school. Broaden what you know in an area. For example, Henry 8 started the English Reformation. Learning in the context of prior knowledge and personal experience.
So learn everything perfectly and not just what is mentioned in class. Connect new learning to past learning. Learn vocabulary by reading and consulting a dictionary, eve think of a few synonyms. Refresh old math skills such as measures. Look at books by Dana McKellar that are cheap on Amazon even if you are a boy. Just learn until learning is automatic. Practice writing. It isn’t how people talk. Its how you think and answer questions INA way that makes sense to a listener.
I would get my learning broad and solid and do test prep to hone skills in answering test questions. Of if you don’t know an answer soon, skip it and leave its item unmarked. With extra time, GI back and complete those items. Personally, after I was sure of an answer and had verified that I had marked the correct answer on the correct answer line. I never looked back. Even when very nervous about a professional licensure service, I relied on my brain. To take the test successfully.
Be of stout heart and head!
I’d take the time to do a timed sample test of each when you have time, and pick the format that works better for you to focus on. I would plan on taking it the last time spring of your junior year but make sure you have at least one more opportunity after your test date to take it again.
One thing that was helpful for my kid right away was just adding a few minutes of math review to his day, you could start with that. My junior is taking hopefully his final ACT next month and is working with a tutor now and that has been helpful but you can easily do the same practicing on your own if you are disciplined.
Don’t underestimate what another year of maturity and quality academics will do for your score.
Might as well start… If you have time, start searching for the resources you want to use for prep.
Thank You!
I will start preparing right off this summer as well as pay strict attention in class 
Agreed with the others. I started preparing the summer of sophomore year and then took the test immediately after returning to school as a junior.
The summer after sophomore is the best time (perhaps the latest for some) to prep for the ACT and SAT. By the first semester of junior year (which is right after that summer), you should be ready for the PSAT which is quite important for merit scholarships at many schools. Also, it would be ideal to finish taking ACT/SAT by the end of junior year (although now there are one summer session for each test), so that you can pick schools on a list accordingly and plan for college visit and look up school information and essay prompts. As there are AP exams in May and you may want to take the corresponding subject test in June, you should plan on getting the ACT/SAT done by April. For both my daughters, they took the first test (ACT or SAT) in the first semester in Junior (before school works get busy) and finish the retake before April if needed.