Since you're going to build a new application and this system, I would like to stop you here and recommend an already build service. Disqus can be embedded in your application using their API.
Please read their API here
] and use it in your applications, it will shorten the time of yours and the data size that would be consumed.
If you still want a native feel, and want to create a commenting system, then have a look at the Database systems. You can create a table to store all the comments by the users, store the UserID (user who posted the comment), Message, Time and any other flag that you might want to save. Then extract the comments from the database using the SQLite data classes for Android.