The process to join a modelling agency in Delhi should be similar to what you will find almost anywhere in the world.
Find reputable modelling agencies, visit the websites of modelling agencies in Delhi. Check to see if they are active in business. Large agencies recruit models many times in a year and they provide consistent work for you but a small agency gives you more personal time so consider this before making a choice on what you want.
If you are in Delhi, you can visit some of the agencies that you will like to join, you can get more information if you visit their office in person.
Read their submission guildline so you know the necessary materials to submit and how to submit it (whether to mail it or to send it electronically).
If the agency is interested in having you, they will call you for an interview, after the interview, it is possible that you join the modelling agency.