I need a little tip here:

If I got an app with my own user database where I create users and stuff and I want to handle them on the device what's the best or the securest way to do this?
I know methods like preferences, sqlite databases or authorization tokens but whats the best of them?

And something else:
If I want to use gmail accounts to login what, which and where should I store the data?

Hope someone can help me ^^

Greetings Franz