USA

Hi Rajesh,
Ideally, the application process including taking the SAT etc should start a year ahead of the intakes, US universities mainly have two intakes. The Fall intake which is in Aug/Sept every year is more common. If you desire to study next year you have to submit your applications by this year end as per the university deadlines. However since you are in 12th it is a crucial stage for you and you must focus on your board exams. Prepare for your SAT too simultaneously and take part in lots of extra curricular activities. Universities in US prefers students with good academics and extracurricular activities such as sports, arts, volunteer work etc. Don't worry about a gap period or so, prepare a good application slowly and steadily and apply on time. You must take the TOEFL too. For more info on SAT check collegeboard.org

Hi Chandrakant,
For MS courses in USA applicants are required to hold a four years bachelor degree and take the GRE. If you don't have a 4 yr degree you have to check with the universities directly for a WES evaluation. You must also take the TOEFL. Check for admission requirements on university websites and apply on time. It is important that you apply before the deadlines. If your application package meets all the requirements you need not worry about the visa.
Shortlist your universities carefully after checking admission requirements. Don't apply to highly competitive universities if your profile is not too strong. For the visa be very clear with your goals and intentions. Visas re mostly rejected if you are not clear with your own plans and if you plan to immigrate. Be confident.