tia,
I started GTD with Excel. I had one file with a separate tab for each context: office, home, errands, waiting for, someday/maybe, etc.
There's not much more to say about it. It takes about 2 minutes to set up. I kept it open all day long.
Do you have any specific questions?
I now use Outlook tasks with the GTD add-in and a Palm Tungsten T2.
moises