How would I go about adding a GUI to a really simple C++ program I've written? I've done a lot of programming in the past, but it's always been command line based and I wanted to branch out to graphical stuff as well.

i know kdevelop has some tools for that if you're using kde.

There are different libraries which will allow you to implement GUIs however (in my opinion) one of the simplest to use and a very well documented option is Qt. This example just displays a window:

#include <qapplication.h>
#include <qmainwindow.h>

int main(int argc,char **argv)
{
QApplication a(argc,argv);
QMainWindow w;

a.setMainWidget(&w);
w.show();
return a.exec();
}

The trolltech documentation site(http://doc.trolltech.com/3.2/index.html) has a number of tutorials and docuumentation about all the classes available. As a further bonus extending to use KDE specific functionality is very easy.

Will what you suggest work in GNOME or would I have to use different header files for it?

You should try glade. It's a easy way to create GUI's using the Gtk library. Although it doesn't compile directly code for C++, it is generated by gtkmm, and you can set your glade for that.

The code will run under GNOME however will have the Qt theme as opposed to the GNOME theme. Similarly a program developed using the GNOME libraries would still look like a GNOME program in KDE. This shouldn't prove a problem and provided you have Qt installed (potentially may require devel packages) compiling shouldn't present any major difficulties. I generally compile with the command along these lines:

g++ -o app -I/usr/lib/qt3/include file.c -L/usr/lib/qt3/lib -lqt-mt

You can use some GUI library:QT,gtk,minigui,fltk,microwindow and so on

gtkmm, qt, and wxwindows. Use one of these C++ GUI library because the support is good and they are popular.

I installed QT into a directory called QT3. How do I compile a program using the qt header files? When I try to, I get no such file or directory for the .h files I'm trying to include.

You should work as much as possible with designer-qt whenever it concerns gui. The simplest project will be if you create the ui file and add a main.cpp file like the one above.
Qt tools, such as moc will generate later a .h file for it, so you can suppose it's already there, how? you will subclass the base class which is the form this ui file represents (includes).

So let's see what we have: some cpp, h, ui, ui.h and a main.cpp file that are located in some folder.
You run => that will make a new pro file.
Then you run that will generate the make file
And the last one of course:

What's a ui file?

Nevermind, I've got it working. Thanks

You might also have a look at TOAD
http://www.mark13.org/toad/