Objective C code for iPhone app

hi I'm writing a code for an iPhone app in objective C and i want my button called SignUp when pressed by user to send alert message as shown is "-(void) registerNewUser" how can i do it? also how do i make it check for existing emails?? Thanks

#import "SignUpViewController.h"

@interface SignUpViewController ()


@end

@implementation SignUpViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    self.Password2.secureTextEntry=YES;
    self.ReEnterPassword.secureTextEntry=YES;
}

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

/*
#pragma mark - Navigation

// In a storyboard-based application, you will often want to do a little preparation before navigation
- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender {
    // Get the new view controller using [segue destinationViewController].
    // Pass the selected object to the new view controller.
}
*/

- (IBAction)SignUp:(id)sender {
    //check if all text fields are completed

    if ([_Email.text isEqualToString:@""] ||[_Password2.text isEqualToString:@""] || [_ReEnterPassword.text isEqualToString:@""]) {
        UIAlertView *error = [[UIAlertView alloc] initWithTitle:@"Error" message:@"You must complete all fields" delegate:self cancelButtonTitle:@"OK" otherButtonTitles: nil];

        [error show];
    }
    else  {
        [self CheckPasswords];
             }
}



- (void) CheckPasswords {

//check that the two apssword fields are identical
    if ([_Password2.text isEqualToString:_ReEnterPassword.text]) {
        NSLog(@"Passwords match!");
        [self Agree];//call Action;
        }
    else {
    UIAlertView *error = [[UIAlertView alloc] initWithTitle:@"Error" message:@"Passwords do not match" delegate:self cancelButtonTitle:@"OK" otherButtonTitles: nil];

    [error show];
        }

}
- (IBAction)Agree:(id)sender {
    if(_Agree.selectedSegmentIndex == 0){
        [self registerNewUser];
        }

    else{ UIAlertView *error = [[UIAlertView alloc] initWithTitle:@"Error" message:@"You must accept Terms and Conditions" delegate:self cancelButtonTitle:@"OK" otherButtonTitles: nil];

        [error show];
;}
}

- (void) registerNewUser {
    NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
//write the username and password and set BOOL value in NSUserDefaults
    [defaults setObject:_Email.text forKey:@"Username"];
    [defaults setObject:_Password2.text forKey:@"Password"];
    [defaults setBool:YES forKey:@"Registered"];
    [defaults synchronize];
    if(_SignUp.buttonType == 1){

    UIAlertView *success = [[UIAlertView alloc] initWithTitle:@"Success" message:@"You have registered a new user" delegate:self cancelButtonTitle:@"OK" otherButtonTitles: nil];
        [success show];
        }


}

@end