clrscr() is not a standard function.  Visual Studio does not have it.  However, MSDN does document how to clear the screen using system("cls"), or FillConsoleOutputCharacter() and FillConsoleOutputAttribute().
As for the cin/cout errors, you need to prefix them with the std:: namespace qualifier, eg std::cin and std::cout, or use a separate using namespace std; statement in your code below the header #include statements.
Try this:
#include "stdafx.h"
#include <iostream>
#include <cstdlib>
#include <conio.h>
void clrscr()
{
    std::system("cls");
}
int main()
{
    clrscr();
    int no;
    std::cout << "Enter a number";
    std::cin >> no;
    getch();
    return 0;
}
Or:
#include "stdafx.h"
#include <iostream>
#include <cstdlib>
#include <conio.h>
using namespace std;
void clrscr()
{
    std::system("cls");
}
int main()
{
    clrscr();
    int no;
    cout << "Enter a number";
    cin >> no;
    getch();
    return 0;
}