What are some things to ensure for a program's security?

I am new to programming. and I wanted to see how I could enhance the efficiency of this program, in terms of conciseness and security.

Also, how can I ensure that the user will not break the program by typing a character or a special symbol when I have asked for a number?

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <conio.h>
#include <math.h>

int main()
{
    //Declaring Variables
    float num1, num2;
    int op;
    char ans;

    //Getting the user to choose an option
Again:
    printf("\n\t\tMENU FOR OPERATIONS:\n ");
    printf("\t\t____________________\n\n");

    printf("1. Addition\n");
    printf("2. Subtraction\n");
    printf("3. Multiplication\n");
    printf("4. Division\n");
    printf("5. Modulus\n");
    printf("6. Power Function\n\n");
    printf("Enter the MENU option: ");
    scanf("%d", &op);

    while ( op != 1 && op != 2 && op != 3 && op != 4 && op != 5 && op != 6)
    {
        printf("\nYou entered an invalid MENU option!\n");
        printf("Kindly try again.\n\n");
        printf("Enter the MENU option: ");
        scanf("%d", &op);
    }


    //Getting the numbers from user
    printf("Enter the first number: ");
    scanf("%f", &num1);
    printf("Enter the second number: ");
    scanf("%f", &num2);

    //Implementing a switch statement for processing the input
    switch(op)
    {
    //Case for Addition function
    case 1:
        printf("Result = %.1f\n",  num1 + num2 );
        break;

    //Case for Subtraction function
    case 2:
        printf("Result = %.1f\n", num1 - num2);
        break;

    //Case for Multiplication function
    case 3:
        printf("Result = %.1f\n", num1 * num2);
        break;

    //Case for Division function
    case 4:
        printf("Result = %.1f\n", num1 / num2 );
        break;

    //Case for Modulus function
    case 5:
        printf("Result: %d\n",(int)num1 % (int)num2);
        break;

    //Case for Power Function
    case 6:
        printf("Result: %.1f\n", pow(num1, num2));
        break;

    default:
        printf("You entered an invalid operator!\n\n");
        break;

    }
    //Asking the user if he wants to run the program again
    printf("\nDo you want to continue (Y/N)? ");
    ans = getche();
    ans = toupper(ans);

    //Implementing a while loop to get the user to enter a valid character
    while((ans != 'Y' ) && (ans != 'N'))
    {
        printf("\n\nYou must type a Y or an N!\n");
        printf("Do you want to continue (Y/N)? ");
        ans = getche();
        ans = toupper(ans);
    }

    //Implementing an if statement to ascertain whether the user wants to continue or quit
    if( ans == 'Y')
    {
        system("cls");
        goto Again;      //links the program to the start in case of a YES
    }
    else
    {
        exit(1);              //Successfully exits the program in case of a NO
    }
}