Authenticate screen xcode 5 ios 7 - ios

Question

I am new in ios and I am stuck for days about how to make a login screen and let the user connect to the application.
Can anyone help me or give me a tutorial because I'm searching for days without finding anything. I am getting my user from a web service.
-(void)listeDesUtilisateurs{
AFHTTPRequestOperationManager *manager = [AFHTTPRequestOperationManager manager];
[manager GET:#"http://127.0.0.1:8888/services/user.php" parameters:nil
success:^(AFHTTPRequestOperation *operation, id responseObject)
{
NSLog(#"JSON out : %#",responseObject);
userTab = (NSArray *) [responseObject valueForKeyPath:#"items"];
NSLog(#"nombre des elements : %d", userTab.count );
for (User *user in userMutTab) {
[user.managedObjectContext deleteObject:user];
}
[self.managedObjectContext save:nil];
for (int i=0; i<userTab.count; i++) {
User *user = [NSEntityDescription insertNewObjectForEntityForName:#"User"
inManagedObjectContext:self.managedObjectContext];
NSDictionary *consultationDic = [userTab objectAtIndex:i];
user.nom_utilisateur = [consultationDic objectForKey:#"username"];
user.mot_de_passe = [consultationDic objectForKey:#"password"];
user.nom_prenom = [consultationDic objectForKey:#"nom_prenom"];
user.nom_prenom_en_arabe = [consultationDic objectForKey:#"nom_prenom_arabe"];
user.specialite = [consultationDic objectForKey:#"specialite"];
user.specialite_en_arabe = [consultationDic objectForKey:#"specialite_en_arabe"];
user.gsm =[consultationDic objectForKey:#"gsm"];
user.telephone_cabinet = [consultationDic objectForKey:#"telephone_cabinet"];
user.adresse = [consultationDic objectForKey:#"adresse"];
user.adresse_en_arabe =[consultationDic objectForKey:#"adresse_arabe"];
}
NSError *error;
if (![self.managedObjectContext save:&error])
{
NSLog(#"Problème d'enregistrement : %#",[error localizedDescription ] );
}
}
failure:^(AFHTTPRequestOperation *operation, NSError *error)
{
NSLog(#"Error : %#",error);
UIAlertView *alert = [[UIAlertView alloc] initWithTitle:#"error"
message:[NSString stringWithFormat:#" %#",error]
delegate:nil
cancelButtonTitle:#"OK"
otherButtonTitles:nil];
[alert show];
}];
}