When GNOME becomes stable there will be a simple recipe for starting up a GNOME session: users will simply type gnome-session and it will start up their environment. But we are not yet at that point: right now UNIX users usually run their stable X Window environment with their favorite window manager, and the run the GNOME panel and other programs from the shell or from their .xsession or .xinitrc files.
I assume that you have the GNOME programs and libraries installed on your system, and that they are in your path.
Startup sequence without session management
start your normal X Window setup, including your window manager
run panel &
You can now start programs from the panel's main menu (which you get to by clicking on the foot icon and selecting the program or submenu. You will usually start up a file manager (gmc) and a help browser (gnome-help-browser) near the start of your session.
Startup sequence with session management
start your normal X Window setup, including your window manager
Run gnome-session. This will start up the basic GNOME programs, like the panel, a help browser and the file manager.