USA

Hi, universities will open the admission rounds after August so you have to keep checking, deadlines differ from university to university, since you will be completing your exams in March 2018 you will have ample time to prepare and apply for the Fall 2019 intake. 6 to 10 months is generally required to prepare for the admission rounds. Start preparing early so that you can apply early. Ivy Leagues are not just about passing your exams with high grades but you must be an all-rounder to fit in. Take part in extracurriculars, do voluntary works, projects etc. See on what ways you can compete with other applicants. You can take the SAT this year so that you can focus on your boards. Or immediately after your boards.

Hi Nilay, what you should study should be known to you, if you still haven't decided then I think you are not prepared to go abroad yet. If you are confused with the subjects then find out your aptitude, likes and dislikes, favourite subjects, most hated subjects etc. There are many courses offered by universities in US, you can browse through university websites and find relevant majors etc and also you can choose your major later.
To get a scholarship you have to do well in your academics, SAT etc and must also have extracurricular activities. Target a SAT score of 1400 and above. Start researching on university admission requirements and prepare accordingly.