How to effectively prepare for college?

One thing to keep in mind is that there are no "recommended activities" to get into highly selective schools like Yale/Harvard.

The only tip I can give you is to pursue 1-2 activities at the highest level possible. Don't go for the quantity of your EC. Go for the quality.

Here is the order of priority of selection criteria that you should aim to achieve-

1) Academics - Straight A's. Steady improvement throughout high school. Good SAT scores (I'd recommend at least 1500). Achieve these and you will at least cross the academic threshold.

2)Extracurriculars- Do things that you are passionate about (Maybe something related to Math/AI/Eng). Leading a math club or being the chief editor of your school newspaper won't be enough. Remember, these schools are highly selective. You will have to show that you truly are unique. Things which I can think of are to have multiple published books or winning Math Olympiads (Not really an EC) or leading some National/International Math Conference. Again, no set EC will guarantee you admission to these colleges. Just do your thing and take it to the highest level possible.

3)Other parts of the application-

Essays, LORs, etc are largely overlooked but very important. Essays can be the make it or break it part of your application. So focus on these if you're planning to apply soon.

Seems to me that if you want to study English at Harvard or Yale than you should get better at checking your grammar.