Before you write the first line of your story, you'd better have at least a pretty good idea of what the last line's going to be.
Before you even get started, you'd better know where you're going.
Confidence is good, but don't assume you're good enough to simply write your way through a story without having any idea where you're headed. I don't know any writers who are. It's okay to let your characters guide you, to some extent. It's certainly okay to give them the room to breathe. My favorite moments to write and usually to read are the little moments, the exchanges, the lines of dialogue, the gritty little character bits. But ultimately, a story is only as strong as its framework.
A solid outline is a writer's best friend.
Usually, you won't have a choice. Most likely your editor is going to want to see an outline. The less they know and trust you, the more detailed they'll want it to be. The bigger and more high profile the project, the more comprehensive it has to be. They'll want to see how one beat leads to another, how things escalate from act to act, how everything ultimately wraps up. The whole thing, soup to nuts. That doesn't mean you have to beat the story until it's dead and work all the spontaneity out of your work. You always want to leave yourself room to improvise, to change things up when you can. The best ideas are often ones that come to you in the spur of the moment. But you better at least have a strong roadmap to guide you on your journey, or you'll undoubtedly wind up lost and floundering.
Writing outlines is not fun. Not for me at least. Go ahead and accept that right off the bat. Writing characters is fun. Writing dialogue is fun. Writing outlines is too much like actual work. Like putting a huge puzzle together. Maybe you enjoy putting puzzles together. I never really have. But I've come to appreciate that I'm always at my best as a writer when I force myself to craft a solid outline, even if I'm not required to.
If you don't know the basics for structuring a narrative, then you first need to learn. Read Robert McKee's "Story." McKee is often criticized, but his book is still something of an industry standard among screenwriters, and applies perfectly to comics as well.
I believe that some of the screenwriting programs like Final Draft have features that can help you in your outlining, though I've yet to check them out. Once you find a process that seems to work for you, it's hard to break it and try something else.
I usually start with a file on my computer. A Word document. Or maybe a physical notebook. Just something I can throw ideas into. Scenes. Character arcs. Themes. Names. Maybe a line of dialogue. Everything and anything that comes to mind, I write it down. And later, from that, I start to build something that bears some semblance of coherence. I break the story into acts. Break it into issues. Move scenes around. I've heard of some writers who use notecards. They write scenes onto the cards and them shuffle them around into the best order. I haven't tried that yet, but the idea is the same.
However you get it done, the point is just to do it. Not just for your benefit or your editor's, but remember, somebody has to draw this too.
I admit, it's fun to sometimes write yourself into a corner without any idea of how you're going to get out. It's a nice challenge. But what you don't want to do is end one issue with your hero trapped in a burning building, weaponless, surrounded by goons, and then open the next issue with him suddenly whipping a baseball bat out of his back pocket and beating his way to freedom, when you never previously mentioned to the artist, "Oh, and be sure to draw a baseball bat in his back pocket. That's gonna be important later." Don't leave your artist out to dry. They don't like it when you do that.
And remember, as I've mentioned before here, if you're writing an ongoing series, chances are you'll be writing for multiple artists. And that will likely require you at least part of the time to write out of sequence. You have to keep multiples artists working at the same time, so if one artist is drawing your first arc, you're also gonna need to jump ahead and write the second arc for the next artist. So in that case, you most definitely better have a solid outline. When you're writing issue #6 before you've written #5, you better damn well know how you're gonna stick your landing in that previous arc.
Despite what you may have heard, I assure you there is no such thing as simply following your muse. No such thing as merely sitting back and allowing your characters to guide you where they want to go. You may get that in very brief moments, in magical little glimpses. The entire rest of the time, it requires foresight and work.
Stories don't coalesce by magic. They have to be crafted.
Does that mean a great outline automatically makes for a great story? No, of course not. It still has to be executed. But like a team going into their big game, it gives you a solid gameplan.