Try to take the most challenging courses at your school. No matter your background, don't feel intimidated by what other people tell you. In high school, I wasn't sure whether to jump to the Honors math track after being told by the Head of the Math Department at the time that I wasn't ready. I took the class anyways and while it was a lot of work, I was able to do well by working closely with my teacher. This story has been a very popular one for summer programs I applied to! Also, if you are going to take less than 4 years of a subject, make sure to check the course requirements for colleges you're interested in.
For clubs, the three things I prioritized were my interest, the time commitment, and the potential for leadership. You want clubs that you are interested in and can get leadership positions, but don't take an overwhelming amount of time (e.g. 6+ hours/week, long weekends, etc. unless you're super passionate about them and can also do well in other clubs + classes). You also want to make sure that your clubs/activities align with your "theme" for college apps. I was interested in STEM and joined the Math Honors Society and Science Olympiad. But, you don't have to do all STEM-y clubs, even if you know your "theme" is STEM. For instance, I was an Editor for the paper and VP of a charity club. What helped me was having a list of the 3-5 clubs/activities I was in or planned to join and the positions I planned to get from those.
In addition to leadership, it's great to find programs in which you can be recognized for your work. For instance, I was a competitor in the NAACP ACT-SO Competition for black high school students, during which I won state medals for my math projects. There are also writing competitions that you can apply for online, like the Scholastic Art & Writing Awards. Make sure to add awards/accomplishments to your list of clubs (see above)!
IN SUMMARY: Make sure to challenge yourself with courses and work hard to do well in them! Find clubs that you're interested in, not too much of a time commitment (unless you're super passionate and can manage it with school + other clubs), and you can get recognized either through leadership positions or awards!
You don't have to do a program the summer after your freshman year, but it's great to start the summer after sophomore year. Reach out to your guidance counselor early in the school year and explain to them your interest in summer opportunities. They can give you some recommendations or turn you to someone who knows more about programs. Make sure you tell them the types of summer programs you're interested in (e.g. STEM). You can also google something like "best summer programs for juniors."
Most of these programs are expensive. Sometimes, programs say that they have financial aid, but it's very little. If you're interested in a program, reach out to them via email and ask about the maximum financial aid before you apply. These programs are also competitive, but don't overwhelm yourself by applying to more than 5 unless they're easy apps. I applied to 2 for my sophomore summer and 4 for my junior summer. Most of these programs also require essays (ask for advice from family & peers) and teacher recommendations, so work on them during your school breaks if possible.
For SAT prep, the summer after your sophomore year is a great time to start. The strategy for increasing your score is taking practice tests and reviewing them. Ask yourself why did I get this problem wrong? How can I improve for next time? Keep practicing until you've eliminated these mistakes. Keep a notebook to track the mistakes you're making on your practice tests and during your studying. For reading, use Khan Academy and borrow prep books from the library. Remember, each answer choice is different! For writing and language and math, use prep books and practice on Khan Academy. Over the summer, take practice tests every week or two to evaluate your progress!
IN SUMMARY: Look for summer programs with strong financial aid and start applying in advance! Studying for the SAT is a marathon, not a sprint. Study, take practice tests, and review them in depth.
Try your best to visit the schools you're interested in (but check their financial aid first!), especially those that track "demonstrated interest." When you visit, pick up all the resources provided and pay attention to things that stick out to you (these will be important conversation topics for your interview!). If you're not sure about your major, look for universities that don't have different colleges (e.g. College of Engineering, Business School) within them unless it's easy to switch or you are interested in only one college. It's also helpful to reach out to upperclassmen or alumni from your school for more information on particular colleges. There are also college guide books that describe schools in depth, and make sure to attend info sessions at your school to learn more about different colleges.
For recommendations, choose teachers who know you as a hard-worker and strong student. It's okay if you weren't the top of their class, as long as they saw you work hard. If you're interested in STEM, it's best to ask your math and science teachers. However, some schools may require a recommendation from your English teacher, so keep that in mind.
For essays, it's a great idea to finish your Common App Essay (or equivalent) over the summer before senior year. In your essays, you want to blow the admissions officers away with something (e.g. a lesson you learned given your background and experiences). Situations in which you are vulnerable usually work well. Starting with an anecdote is great too. Remember, show don't tell and make sure your writing is the best it can be (i.e. anecdotes are descriptive enough for the reader to imagine what's going on). I started off my essay with an anecdote about how my aunt's neighbors and family prepared her wedding in Eritrea. I contrasted this Eritrean community with American individualism, which was present in my private high school. My big takeaway was that when I go to college, I want to strive for individual success and build community. Make sure to get at least 2-3 people to read your essays!
IN SUMMARY: Visit schools if you can, choose teachers who know you work hard and do well academically, and start essays in advance!