C++ Program:

Example:

#include <iostream>
using namespace std;

int main() {
  int num;
  long long fact=1;
  cout<<"Enter your number : ";
  cin>>num;
  cout<<"Factorial of "<<num<<" : ";

  for(int i=1; i<=num; i++)
    fact*=i;
  cout<<fact;
  return 0;
}

Output:

  Enter your number : 3
  Factorial of 3 : 6

You should know first :

Factorial of N : 1*2*3*4.....*N
Factorial of 0 : 1

The variable which holds factorial value (above fact) must be of type long long

Because,
long long : Factorial of a number can be very large [valid for upto 20!]

for(int i=1; i<=num; i++)
  fact*=i;