I am a science student and wish to persue IAS. The best way to persue??

Hello student,
It doesn't matter from which stream you come from. If you have completed you class 12 right now, them in your graduation, go for some humanities related subjects like political science, sociology, history, economics of geography. This is because, these subjects have a huge overlap with your GS papers and thus choosing your optional too becomes easier. Next, there are two ways of preparation - one is through self study and the other is to join any coaching institute. Merely joining a coaching institute doesn't ensure your selection. Hard work, sincerity, dedication, perspiration, time management - all matters here.
Good luck