First, be sure you are reading. Both comics and novels, anything with a story. Pay attention to how other writers introduce characters, plot, places, as well as build up, climax, end. See how published writers handle everything, and pay attention to both what you as a reader feel that works and doesn't. Talk with friends and peers of these storytelling elements. Studying story helps with writing story.
As far as physical writing, these are steps I like to take when writing.
1.) What is the plot summed up in one sentence?
ex: Character A is hungry and makes a sandwich.
With this, you've got your focus character, their goal, and why.
Then build on it. What is keeping them from meeting their goal? Perhaps there is no bread, so they must go to the store. How do they handle this? Compile a list of different preventions that get in the character's way. How do they handle or over come each obstacle? With this list done, you then can pick and choose what you feel builds the story the best. There's nothing wrong with throwing away an idea if it doesn't build the story up. Finally, figure the end. Does the character succeed in making the sandwich, or does the character fail?
2.) With your rough ideas laid out, compile bullet points. I personally like to seperate them by scene.
This allows you to see the story laid out, and figure the best flow of events. Also if you're planning for a short story, or even separating it into many chapters, it helps with the pacing. You can determine the best place to end each chapter, and start thinking of the story by pages.
3.) Finally, roughing your script. Using your bullet points as a guide, you can start writing. Something to be mindful of, make sure each page builds on the story in some way. There should never be a page in which nothing productive happens. This can be done in many ways. Introducing the character's world or building upon it, introducing a character, a disagreement or agreement happens, etc.
4.) With your story finished, begin going back and tweaking it. Personally, I like writing months to a year in advance, that way after I write, I can step away from it for a few months, and come back fresh. Fixing the story with a fresh mind helps you see many of the flaws from the initial writing. Continue to fix upon it.
5.) Once you feel its done, find someone(s) who doesn't know the story or characters read it. They can tell you on places where they got confused or felt that something was not explained well enough. The fact that they do not know the characters or story helps you to fix problems before you start the art phase. The fact that you, the writer knows every aspect of the character is good, as you can fully show them to the reader. But the weakness in this is that not every aspect of a character is told in a story, and this can cause confusion if you hint at something the was never previously explained.
6.) After that, keep fixing upon it, until it is as good and solid as it can be.
Hope this helps you.