I started to do Getting Things Done in September of this year, and I love it. It has helped me get rid of a million little things that distract me, and thereby improve my focus upon what is important. I wanted to quickly share some getting started advice. These are the major lessons I have learned as I have been developing my own system.
Start implementing the GTD system immediately. Do not wait. Anything is better than nothing. If you don't own the book, order it right now. In the meantime, do an internet search for "basics of getting things done", write down a few policies to implement, and start. The "system" rewards time invested in it. Every bit of time you invest helps, and this is especially true over the long run.
Buy the book and read it. There is understanding that is essential to the system which the book teaches you. If you do not understand it, your system will not work. Getting Things Done cannot be effective only as bulleted list of principles or flow chart of rules. You must understand the theory to be effective.
The GTD system is your system. It's just a set of principles. How you implement them depends upon you and your current situation.
Do not let perfect be the enemy of better. Expect your system to get a out of date and have rough edges. Refactor and iterate.
Moreover, there is no such thing as perfect.
Pay attention to what feels difficult. This is what you need to work on. This difficulty is valuable for you to improve.
Getting started is much harder than maintaining. When you start, you will have an infinite list of projects. The good news: odds are that you will get a bunch of them done quickly, and most of the others will go onto someday/maybe.
Do not be afraid to put things onto someday/maybe. For me, it was probably the single most valuable part of the system. Remember that "someday" can mean "I just can't act on this right now", be that because of restrictions with time, money, or health.
Getting Things Done does not replace your own agency. You are still your own master. The system is merely a set of insights and practices that help lots of people keep their focus on what is important.
Finally, try to implement one idea at a time. Don't try to make huge, drastic changes all at once.
The main point is to keep trying. If something seems a little hard to you, try to think of a way to make it easier. Keep going back to the book, think about the principles, notice things you can do to improve, and keep trying.