how do i design/architect my user details app in iOS

I'm new to iOS and have just started leaning it. I want to develop a small app for bus passengers where-in all users in the bus must login using the app.

If user is using the app for 1st time, he must sign up . User must enter all details like Name,Age,Emergency Contact,Address,Source,Destination,Phone number etc.

If user already exist, then login with existing user name and password.

All details must be stored somewhere (not sure) and retrievable format.

Here comes all my question and doubts based on above app requirements :

1. do i need to follow client-server architecture ?(mobile app being client )
2. where all user details will be stored ( on mobile or server )
3. when user tries to login, how check if user already exists or not ?
4. if any communication protocol to be used for mobile app communication then which will be good considering the performance of app should be fast.
5. mobiles internet data should be available ?
6. which database to use to store user details ?

considering all above things i need to design my app.

thanks