Here's how I got started, pretty much: I got a (paper) notebook for GTD and started carrying it around with me everywhere. It can have, for example, one page for "next actions -- at home", other pages for actions in other contexts, maybe some pages for particular projects, etc. Also a calendar to carry around with you.
Whenever you happen to think of something you need to do, you can either write
it immediately on the appropriate "next actions" page, or else write it on a
general-purpose "capturing" page, to be processed later. You can keep "capturing"
pages in a few places, such as beside your bed.
You can schedule a "weekly planning" on your calendar, and when that time comes,
then collect all your "capturing" pages and process them; make a simple list of
projects (what do you want to get done in the next week or so?), and maybe some
lists of actions towards some of the projects.
I got into GTD gradually. Over a number of weeks, my GTD notebook gradually
contained larger proportions of all my planning stuff. I continued whatever systems
I was using before unless or until I didn't need them any more. Some of my
previous systems, such as a calendar I carry in my pocket, became part of my
I used little bits of time here and there, such as while riding a bus, to write things
in my GTD notebook.
If you try to make it too big, all-encompassing and perfect, you might never start.
You can start small, as I did. David Allen suggests taking a whole weekend and
re-organizing all your stuff. That may work better for people hiring him as a
consultant but may not be necessary for people working from his book.
Inability is an abstract thing involving comparison with alternate universes; it cannot be experienced.