Well, you're the only one who can think up of your idea. If we thought up of an idea for you, it wouldn't be yours. Just some other guy's/gal's idea. It's alright if you think of your own idea and we build onto it with more ideas.
If you're trying to get an idea, you just have to let them come. Then that one idea will build up and up and up. Of course you have to play your cards right for it to be a good story.
To try to hatch an idea, watch a movie, read manga, do anything that is entertaining. The best ideas are the ones that are unexpected, not forced. So don't force yourself to think of an idea.