How to setup Arduino on secure wifi network?
How you tell the WiFi shield how/what to connect to is purely down to the shield.  There will probably be a library, and that library will have functions you call.

How you then get that data from the user is entirely up to you.

Yes, an LCD and buttons is probably the easiest way.

Either that or you have some other interface to the Arduino (Serial over USB is the simplest as it already exists) that the user uses to configure the system using a PC.  Once configured the system can be disconnected from the PC and will be stand-alone.