Depending on what version of XCode you are using, and compiler there are different ways of doing it. I don't know how familiar you are with OOP, if you are not I suggest you read up a bit on setters and getters and objects as it is the basis of almost everything you will do from now on.
Some examples, Old school style, will create an ivar. In your .h:
@interface TheViewController : UIViewController{
NSString *theString;
}
A bit new style, will create setter and getter In your .h.
@interface TheViewController : UIViewController
@property (nonatomic, weak) NSString *theString;
In your .m file:
@implementation TheViewController
@synthesize theString = _theString;
Can be accessed by _theString or self.theString
The new way of doing it. In your .h file:
@property (nonatomic, weak) NSString *theString;
The compiler Will create everything the above way did.
Hope that helps you a bit.