I want to use instagram's API but it asks me for authentication. I have logged in instagram but still I cannot get the authentication. Going through the web I found that we need to Register a new client but I don't know what details I need to give there.
This is certainly not a comprehensive answer, but to get you started go to https://www.instagram.com/developer/register/ and look for the Manage Clients button in the upper right corner. You'll need to be logged in first. Then click Register Your Application. You will have to have your callback URL ready to go, what they call the redirect_uri.