Allow app to communicate with web domain

I have a domain that is user posts based. I plan to create a user posts based app like 9gag. I need the app to be able to communicate and fetch data hosted from my domain.

Things I need the app to do:

1) Allow users to post pictures though the app.

2)Allow users to leave comments through the app.

3)Allow users to leave 'likes' though the app.

I want the data to be stored on my domain, while when a user opens the app, the app will fetch this data from the domain and display it for the user. How can I make my app communicate with the domain?

Thanks!