SAT vs ACT and preparation.

I am a ninth grader from India (but a US citizen) who wishes to study in the US for medicine, hopefully in Ivy Leagues.

What is the difference between the SAT and the ACT? Which is preferred and what is the new format like (for SAT)?

Also, what are some resources for preparation. I will be taking a lot of PSATs. In eight grade, i got a psat 10 score of 1390/1520. I really need help with reading and writing. My grammar is exceptional but my analyzing skills need major help. Can anyone reccommend resources for the PSAT and real SAT as well.

Also, what are the norms with subject SATs. How much do they add to a resume and which ones should be taken for biology and medicine streams?

Thanks! Any help is greatly appreciated.