You have a great story, a bunch of characters and you want to create a manga. However, where do you start? Do you have it Left->Right or Right->Left? How should you publish it (*note: some publishers may have additional requirements/restrictions)?
The process is pretty much the same for any graphic progression such as comics.
Luckily, there are a great number of tutorials to help you on your way.
A storyboard is a panel or series of panels which detail the sequence of significant events in a planned scene, i.e. a rough sketch coupled with some words to describe the actions in the scene.
At this point you'd need to have an idea of whether you are making a left->right or right->left manga and your story has been separated into scenes and plot points you wish to use to drive your story.