#include <iostream>

  gives you cin and cout, etc

#include <string>

  needed if you use a string for anything

#include <cmath>

  for the mathematical functions sin, cos, exp, etc

#include <iomanip>

  to control formatting with cin and cout etc

#include <fstream>

  for accessing files

There are many others.

Remember to say near the beginning

  using namesapce std;

and declare instead of void main()

  int main()
  { ... }