Building A Airline Simulation Game Whereby Users Create An Run An Airline?

I'm looking at building a airline simulation game whereby users create an run an airline, I just want to ask something that will possibly be very simple but it's going to be very complicated answer i think.for every aspect of my game do i have a class to create an object, for example

City: (I'll have a class called myCity and each time i look at a city it goes to that class creates an object and I use it then dispose of the object)Airplane: (make model etc) How about bigger items such as Bank is bank it's own class or are items inside the bank the object?

How about competitors, i guess each one is it's own object? i would never dispose of them, how would I store them form each user session to another?