Urgent plzzzz answer iy

Firstly, you need to search for a Firm where there is a vacancy for Article Trianees and then after getting selected, purchase Articleship Registration Forms which are available at the regional centres (you should not download from the ICAI website because Form 102 & 103 available there are only for referance but not for usage; Also Identity card is not available in the website). 

Regarding your other question as to whether you should go for a Big 4 Firm or others, I can say that you should take your own call. You may go through the following link for getting to know the pros and cons of doing Articleship at a Big 4 Firm (https://www.charteredclub.com/my-ca-articleship-experience-of-working-at-deloitte-big-4/)