I want to allow users to stream media from smartphones to TV and Is it requires any registration ?
If yes, So where and how can I register my app ?
Below are guidelines in registering your application.
You must register your application if you are using the Styled Media Receiver or if you are building a Custom Receiver. Once you've registered your application, you'll receive an application ID that your sender application must use to perform API calls such as to launch the receiver application.
If you use the Default Media Receiver, you do not have to register your application. The Default Media Receiver does not allow you to provide any styling to the media player UI, and uses a provided constant as the application ID.
You must also register your Google Cast device so that it may access your receiver application before you publish it. Once you publish your receiver application, it will be available to all Google Cast devices.
The mentioned information and detailed instructions in App Registration for Google Cast is on this link: https://developers.google.com/cast/docs/registration
You can register your application for Google Cast via Google Cast Developer Console. If you are a first time user of the site, it will require you a $5 one-time payment for signing up.
UPDATE:
The application type in registration will depend on how you have implemented the receiver.
Styled Media Receiver is a pre-built receiver application that provides a media player UI for audio and video content. It provides a default UI for the receiver application, but you can customize several elements in the UI with a CSS file.
Custom Receiver is a custom built HTML5 app that you must host to handle the display of your app content on the TV. You may need to create a Custom Receiver if your app wants to display content other than audio/video media or if the Styled Media Receiver does not support the media types your app requires.
If you choose not to implement a receiver, your application can use the Default Media Receiver.
Detailed information about Receiver Applications are in this link: https://developers.google.com/cast/docs/receiver_apps