Just get your preferred drawing tool and sketch page 1. It'll probably suck and you'll probably want to redo it quickly, but you shouldn't. Make page 2. Then page 3, and so on. The best way to improve at making comics is to make comics, and look at what you've made and think about what's good about it, and what's not working, and what you could try doing differently, and then do it differently on the next page/chapter/story.
Normally I'd recommend doing thumbnails (quick, small sketches to figure out the layout of the comic), but when you're just starting out, I think it's more important to just make comics. Start thumbnailing later, once you've figured out how to make a page. You can't really plan how to make something before you have an idea of how to make it xP
If you're really afraid of messing this story up since it's one you've been working out for years, then do a few shorter practice comics with throw-away ideas to practice your visual storytelling skills.
If you like going in prepared, then read Scott McCloud's "Understanding Comics", and then his "Making Comics", which will help you think about some things in comics that you and your readers likely take for granted, so that you can use those things to make the comic better.
There are many other books, but those two are great for getting started, as they focus a lot on what makes comics work the way they do rather than specific techniques.
Another thing you should always do, even when you think you've gotten pretty good, is look at a lot of other comics made by many different authors from all over the world, of all skill levels, in all kinds of genres, and analyse them. Think about why they make the decisions they do. Why is each page laid out the way it is? Why is each panel composed the way it is? Why does the chapter end here and not on another event? Why does this character have this sort of character design? All that kind of stuff. These questions all add up to the real answer you're looking for: what makes these comics work, and what about them doesn't work? Use that to improve your own comics. In other words, learn from other people's mistakes, and from their successes. Once you have a pretty good idea of how to make a comic that works, you can even look to non-comic sources for inspiration and knowledge, since by then you'll be analyse them in terms of how they can benefit your comic-making.
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