Few steps will led u to understand the simple implementation involved in it: 

Step.1-Include  MessageUI.framework in your project.

Step-2 Import  MessageUI/MessageUI.h in your header file like that in my case.

#import <MessageUI/MessageUI.h>
#import <UIKit/UIKit.h>
#import <QuartzCore/QuartzCore.h> 
@interface SendSMS : UITableViewController<MFMessageComposeViewControllerDelegate>
@property (retain, nonatomic) NSString *SMSSubject;
@end

 Setp-3 Suppose you want to send SMS on button  touch(click) then write the following code in your implementation (SendSMS.m) file. 

  MFMessageComposeViewController *controller =[[MFMessageComposeViewController alloc] init];
     controller.messageComposeDelegate = self;
 
// CHECK DEVICE CAN SEND TEXT MESSAGE. 
    if([MFMessageComposeViewController canSendText]){
// message subject. controller.subject=@“this is message subject. ”;
//  message body controller.body=[NSString stringWithFormat:@“ This is the testing message. “];
// contact number you want to send . controller.recipients = [NSArray arrayWithObjects:@” 123456”,nil];  
         
// check the condition in case of model view controller is not dismissed.
                if (![[self presentedViewController] isBeingPresented]) {
                    [self dismissViewControllerAnimated:YES completion:nil];
                 
                }
// show message view  UI.[self presentViewController: controller animated:YES completion:nil];
            }
 
// delegate method of  MFMessageComposeViewController . - (void)messageComposeViewController:(MFMessageComposeViewController *)controller didFinishWithResult:(MessageComposeResult)result
{
      [controller dismissViewControllerAnimated:YES completion:nil];
}