To write to Prime Minister Narendra Modi, use his official link instead of any other Third Party App. They just forward collected data to PM again.
Be positive. If you do not like anything, give suggestions to improve instead of just criticizing. He regularly accepts good suggestions and puts them into practice.
You can also use the same link to ask for a meeting with the Prime Minister. Instead of just a courtesy call, go there with a mission or a project of public interest.