How should I write an essay which is helpful for getting admission in overseas universities?

It is called SOP Statement of purpose. you can get guidelines in shiksha website itself as how to write an SOP. it is statement about you and you can describe how did you get in to this particular area of study and what is your ambition in life. There is also a website called statement of purpose.c

Dear Shrishty, Essay topics are given by the universities at the time of application and you need to frame it that way.You can take help of professional overseas counsellors to help you out. Best Wishes