Synthesis of weak property only allowed in arc or gc mode

Hey i just start ios programming today and i am facing this erro.

plz help me to remove this error

plz suggest me some nice ios developer tutorial

#import "ViewController.h"

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
}

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

@end


#import <UIKit/UIKit.h>

@interface ViewController : UIViewController
@property (weak, nonatomic) IBOutlet UITextField *txtUsername;
@property (weak, nonatomic) IBOutlet UITextField *txtPassword;
- (IBAction)loginClicked:(id)sender;
- (IBAction)backgroundClick:(id)sender;


@end