Visual C++ :: Adding To Pro File (QT)

I created a new project in Qt Creator, the files included are the .Pro file, and the .Cpp file.

Qt does not recognize the headers I try to reference in the .Cpp file. I am pretty sure it's because of what I don't have in my .Pro file. What do I add to it?

.Cpp

Code:
#include <QApplication> ->No such File or Directory
#include <QPushButton> ->No such File or Directory
#include <QHBoxLayout> ->No such File or Directory
int main(int argc, char *argv[]) {
QApplication app(argc, argv);
QWidget *window = new QWidget;

[Code] ....

.Pro

Code:
SOURCES +=
MainWindow.cpp