You can use std::cout each time or you can declare it in either of two ways.
Using std::cout makes the file smaller, but you then have to declare similar things for every std class function you want to use, like cin, etc. The namespace way brings them all in. For more info on namespace & std, check posts for the last week or so,there's been threads on them there.
#include <iostream> // Use this for both
using std::cout; // here's one way
using namespace std; // here's another
cout << "Hello world!";