USA

No, it is not necessary. But, if you have work experience, it will give you enhance your profile.

You must go for USA.

Hello,
Applications for Spring 2018 will be available from March 2017. Australia and NZ have intakes mainly in Feb and July. So the applications will be available from February 2017 itself. Whereas, for US universities you can apply in April 2017 for Spring 2018 intake.
All the best.

Hello,
Please find the link of top universities: https://www.timeshighereducation.com/world-university-rankings/2016/world-ranking#!/page/0/length/25/country/0/sort_by/rank_label/sort_order/asc

I wouldn't suggest you any of these because the living expenses in these countries is just of the league. In my opinion, go for Germany or New Zealand. Germany offers international students scholarships that result to very less fees or even no fees.
New Zealand is also offering many scholarships. The

Your score must be 85-95% in boards for top ranking colleges like, Harvard and Stanford University and also for scholarship.

The higher score the better. 60% is the minimum recommended percentage.

If you can qualify SAT exam which provides you the top university in USA and if your score is good in SAT and TOEFL, you may get 30-50% scholarships. Without any scholarship, maybe the money required is minimum 50 lacs. Mark my words for the top universities.