how to get the top rank in various maths olympiads...?

Answer 1

There are quite a few books that can help. Maybe a local or school library in your are will have some of them

http://www.google.com/search?q=math+olympiad+questions&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a

Answer 2

In google type:

imo-official

When you see list of results click on:

imo-official org

Answer 3

The procedure suggested above by anonymous will let you download previous problems at
http://www.imo-official.org/problems.aspx

Seems like a good place to start.

Answer 4

Here is a list of previous math contests in different categories from grade 7- grade 12 from my old
"Alma Mater" , the University of Waterloo.

http://www.cemc.uwaterloo.ca/contests/past_contests.html

This year, in just the grade 7-8 category, it drew about 80,000 participants.

All contests, results, and solutions can be downloaded.