Creating A Story
Putting together a story is simple in it's bare-bones. Putting in the details is what makes it harder. Let's start off with the general layout of a story.
Intro/Setting - You introduce
the main character(s) and the setting of the story.
Problem - Your character is faced with a problem, sometimes more than one.
Dealing with Problem - Your character goes through certain events on how to solve his/her problem.
Solution - A solution is reached after a setback or two which leads to the...
End - Happy ever after (in most cases)!
Since manga generally can be about anything, here's a full list of genre's in the literary world.
Some additional genre's are: Action, Adventure, Romance, Comedy, Martial Arts, Drama, Western, Detective/Crime, Mystery, Suspense, Adult, War, Political, Magic Girl, and Anthromorphic (aka Furries).
Making a Story
To make a story, here is a basic checklist for you:
4. Story (Outline, Dialogue, etc)
Most importantly, have fun!