I am in 10th grade and I don't know when I should start preparing
You should be starting to apply the summer going into your senior year. However, its always recommended to start touring schools that you are interested in as early as sophomore year, and start to plan your junior year schedule based on the rigor of classes the colleges you're looking at recommend.
Sophomore and junior year are great times to start thinking about the colleges that fit your interest (size, location, prestige, financial aid, etc.). You can do this by touring colleges and I would even recommend going to virtual information sessions that are available on many college websites because of COVID. At these informational sessions, you will learn a lot about the college to see if it's somewhere you might want to apply. Make sure you are staying on top of your grades and try to earn leadership positions/awards in your ECs. You will apply for college in the summer or fall of your senior year, but I recommend starting to build your application as early as possible, by creating a list of all of your ECs, awards, AP classes, GPAs, and overall achievements. You can even start practicing your college essays. There are several mock prompts online that can help guide you on how to write excellent college essays that will wow admissions officers. My advice for college essays is to be as genuine and unique as possible. For standardized testing, try to take both the SAT and ACT at least once - most people are better at one than the other. You should take the PSAT in your sophomore year for practice. After, you can do SAT practice tests (there are practice tests available on College Board's website). You should take the SAT for the first time in the winter or spring of your junior year. I hope all of this helps. Even though you won't be applying to college for a couple of years, it's never too early to start preparing!
To keep this community safe and supportive: