Hi. If you currently don't have an idea for the story, you can try to begin with making some concept arts of your characters. Maybe draw some scenes, in which their personalities or abilities would be revealed in spectacular ways.
If you will do it, it will help to:
1. Specify and polish design of your characters;
2. Improve your art skill, which will be useful when you'll make a comics;
3. Specify the style, in which you'll draw your comics;
4. Get an ideas of scenes, which you will want to include in it;
5. Maybe, eventually, drawing these scenes, you'll also get some ideas for the plot, in which these scenes take place.