Создать метод

-(IBAction)registerNewUser; //регистрируем нового юзера
{
    NSUserDefaults *userData=[NSUserDefaults standardUserDefaults];
    [userData setObject:usernameField.text forKey:@"username"];
    [userData setObject:passwordField.text forKey:@"password"];
    [userData synchronize];
}

-(void)registerNewUser:(NSUserDefaults*)dataBase
{
    
    [dataBase setObject:usernameField.text forKey:@"username"];
    [dataBase setObject:passwordField.text forKey:@"password"];
    [dataBase synchronize];
}

- (IBAction)registerNewUser;
{
    NSUserDefaults *userData = [NSUserDefaults standardUserDefaults];
    [self synchronizeUserData:userData];
}
 
- (void)synchronizeUserData:(NSUserDefaults *)userData
{
    [userData setObject:usernameField.text forKey:@"username"];
    [userData setObject:passwordField.text forKey:@"password"];
    [userData synchronize];
}

#import <UIKit/UIKit.h>
 
@interface newClass : NSObject
 
@property (nonatomic,retain) IBOutlet UIButton *registerButton;
@property (nonatomic,retain) IBOutlet UITextField *userName;
 
-(IBAction) registerNewUser;
 
@end

#import "newClass.h"
 
@implementation newClass
 
@synthesize registerButton, userName;
 
-(IBAction) registerNewUser
{//объявление метода
}
//и ещё кучка методов
@end

#import "newClass.h"

-(IBAction) registerNewUser

#import <UIKit/UIKit.h>
@interface newClass : //(наследуемся тут)
{
UIButton *myButton;
IBOUtlet UILabel * myLabel;
}
 
@property (nonatomic,retain) UIButton *myButton;
@property (nonatomic,retain) IBOUtlet UILabel * myLabel;
 
@end

@synthesize myButton,myLabel;

int main(int argc, char *argv[])
{//объявление @autoreleasepool
}

int main(int argc, char *argv[])
{
    newClass *obj = [[newClass alloc] init];
    
    [obj.myButton doSomething];
    [obj.myLabel doSomething];
 
    [obj release];
}