As soon as you decide to study abroad in the USA start preparation as you will require much time before application. USA universities have some conditions to be fulfilled, check them out and start giving your required exams, collect all the documents required etc. Then after proper research apply to the universities in USA. There are 3 intakes offered in the universities of the USA. They are Fall intake(From September to December), Spring intake(From January to May) and Summer intake(starts around May). Before the deadlines arrive, start applying. You can get more information here: Intakes in USA