Getting into US universities is a long-term process and can be time-consuming. Compared with other English-speaking countries that follow the British higher education system, admissions to most good American universities will be characterized by much more diverse entrance requirements, which means you have to decide where you wish to go and work hard for that specific goal for at least 3 months in advance. For international students, the language requirements at highly-ranked universities will be a huge barrier, so preparation should logically begin as much as one year earlier.