I suspect you are looking at the "GTD Connect" subscription. That won't give you a working system. It will give you access to ongoing education about how to be more effective using a GTD system to organize yourself. There are many people who like the continuing education material, and feel it helps them maintain an effective system. There are many others who effectively use GTD without it.
As Adam suggested, start by reading the book. It will tell you what you need to know about how GTD works, and how to set up a system for yourself. Google will also turn up an amazing number of blogs and sites that will help you implement GTD. One suggestion, begin with the lowest-tech implementation you can stand to use. If you can stand to hand write on paper (I can't if there's any way to avoid it) you will learn how to use GTD most effectively with an entirely paper based system. Then move to something with a higher technology as you find a need for it. IMHO starting with a "GTD Application" is going to teach you to rely on the application developer's decisions, not necessarily the GTD approach.
If you really want to spend money to get started, the GTD Implementation Guide from DavidCo (https://secure.davidco.com/store/catalog/GTD-IMPLEMENTATION-GUIDE-LETTER-SIZE-p-16636.php) is pretty good, and more step-by-step than the book. There are also implementation guides for various standard platforms and tools currently available, e.g. iPhone, Outlook, Blackberry, Lotus Notes, Omnifocus, and so on. I haven't seen any of those, no opinion on how useful they might be.
At its root, GTD is "advanced common sense", as David Allen says. Keep a list of the stuff you're committed to doing. Keep a list of the next thing to be done for each of those. Sort that task list ahead of time so you can grab something off it to be done based on your resources right now (if you're not online, no point in seeing a task that requires visiting a web site, for example). And review your lists often enough to be sure things aren't falling through the cracks. Software can help with making the mechanics efficient, but can't automate the decisions you have to make about what you are going to do.