Well, I still don't have much experience in this field, since I still have to pitch my series, but I did some research for myself, so I hope I can be of help.
First thing you should be doing is: search for potential publishers. What publishing houses would accept your story? You have different options:
1) Viz Originals: If it's a manga, you could try Viz originals. Viz is giving a chance to creators from all the world, to publish their own manga with them. Even if it's not a manga, you can still publish your own work with Viz.
2) Dark Horse comics: This one is a more well-known publisher. It's a little bit hard to pitch your story to them, but you could always give it a shot.
3) Saturday AM: Again, if you wish to publish a manga, you could also try Saturday AM. It's a Shonen Jump-inspired magazine, with works from authors from all the world.
These are just a few examples of publishers I know of and that accept pitches. Then again, you're the one who has to do most of the work. Search as many publishers as you can and then pick the one that you find to be the most promising. Once you do the pitch, though, you might have to wait some time (even months) before an actual reply, and there's no guarantee they'll immediately accept you. Sometimes you have to try more than once and try with different publishers.
Now, for the pitch itself, there are some things you'll have to do. Every publisher has their own set of rules when it comes to submitting a story, but generally they all require:
1) A synopsis of the story, where you'll have to include all the key points of your story (don't shy away from revealing important details and major events, publishers don't mind spoilers!).
2) Some examples of the artwork from your story. If you have a bunch of pages or even an entire chapter ready, you should send everything to them (they will be the ones to specify every detail, for example how should you send it, what format you should be using, etc.).
3) Information about the characters and the world.
4) Potential target. Publishers often want to know what kinds of series are the most similar to yours. It's a way to know what your potential target demographic actually is. Is your story aimed at children? Teenagers? Adults?
And they might ask for more details. It's important, as I already said, not to shy away from details and to reveal them as many things as possible. Pitching a story to a publisher is definitely not an easy task, so don't get discouraged if you get rejected on your first tries.
Here's a few videos that I watched to know more about the publishing process and how to pitch to an editor. Hope this helps 
It's important to also keep in mind that you can choose to self-publish your comic instead. I suggest you see the pros and cons of both self-publishing and traditional publishing. Best part of traditional publishing is you won't have to do marketing, but the downside of it is that you will have to split your income with the publisher. As I said, you should try and pursue the best options for yourself. Wish you the best of luck! 