I have an app that when the user clicks on button, i want to send her/his response to the server, I want to tell the user that that I'm sending now the data to the server.
Thanks
The answer depends on why you want users to know information is going to the server, and how you want to affect their behavior.
If you're just reassuring them that their information didn't vanish, you could go ahead with the spinner in the phone's status bar, or a few-pixels-high progress bar like the one used in Messages.
If you need them to wait until their information is received, then there's no need to be subtle. You can give them a progress bar overlay that actually prevents them from doing anything else in the app until the server confirms receipt. Or, you can distract them by giving them a task that can be done asynchronously (e.g., tagging their recent submission). This post has some good examples of progress bar, asynchronous activity, and dummy content methods.
If you need them to know that the information currently on the screen might be updated but it's still okay for them to look at the old info, you can leave the screen as it is but provide a more conspicuous progress bar or loading icon, similar to the refresh-on-pulldown throbbers used in Mail and Twitter.
What do your users expect? What do apps similar to yours do? Ideally you would know your users and the apps that they use - this would give you a good place to start. Then, if you can, make some mock-ups (or prototypes) and ask your users (or your target audience).
Oftentimes you'll find out very quickly which alternative is better.
