Firstly - welcome to the wonderful world of comics! I hope you'll like it here. =)
My first bit of advice would be to make your first project a very short one - 10-30 pages long - so that you can get a feel for what it's like to make a comic, and see if you enjoy it or not. It's also nice to have a short, finished project under your belt before you take on something larger, because then you know what it feels like to finish a story! It's valuable knowledge.
My approach to planning comics differs a bit depending on whether it's a short comic or a very long one, but the basic building blocks are the same! I've got a load of different documents and notebooks and sketchbooks and whatnot, and working on my comics always means hunting all of them down - you could definitely be more organised than I am!
Step 1: Brainstorm
What kind of comic do you want to make? What kind of things would you like to put in it? This is the no-holds-barred, throw-everything-at-the-wall-and-see-what-sticks step of creation. Really ponder what you'd like to do, and write down anything that sounds interesting! You've already got an idea, so you're fine.
Step 2: Outline.
Write a brief, summarised description of what happens in your story from start to finish. Try to avoid getting too detailed - you just want an overview of the story. This helps you get a grasp on what needs to happen in each part of the story, which makes the next step easier. Also, Knowing the big-picture version of your story and how it ends helps you avoid writing yourself into corners and having to re-write stuff.
Step 3: Page-breakdowns
When I'm working on a longer story, I take a chunk of the outline large enough to make a chapter - and if I'm working on a short story, the entire outline - and then start deciding what needs to happen on each page. The first five pages of my comic Grassblades broke down as follows:
Page 1: Main character walks through woods. (establish setting)
Page 2: He's startled by a bird, and spots something by the roadside.
Page 3: He goes to investigate.
Page 4: He finds a clearing full of dead bodies.
Page 5: He's surprise-attacked by a bandit
Step 4: Thumbnails
After the page-breakdowns are done, I take those and start translating them into thumbnails for each individual page - like the ones @shazzbaa posted; mine are very similar! The difference between her thumbnails and mine (I think?), is that I write my dialogue alongside my thumbnails; before this step in my process, I haven't got any written dialogue.
Planning my dialogue alongside what the pages will look like helps me avoid over-crowding each page with text, because I plan where the speechbubbles go at the same time as I plan what the panels will look like. So this, honestly, is the bit where I write the "script". I don't have a separate, text-only dialogue script; it all goes into the margin-notes of my thumbnails.
Step 5: Drawing the thing
Pretty self-explanatory. Once you've got all your planning lined up, all you have to do is translate those thumbnail-scribbles into actual, full-sized drawings!