You need three basic things for a story:
- Characters that people will care about and remember.
- A convincing location or setting for your story.
- A problem your characters have to solve.
Discovering the solution to the problem (or better yet, problems) is what keeps people turning the pages. Don't describe too much at the start, get the reader involved first and tell them more as you go. I like to know the ending for a story before I start writing - so I know where I'm headed. I can change the ending if I want to, but I'll know why I'm changing it.
Here's an idea to get you started: A boy and girl come in to their school one morning and find it empty. It's a big school, with loads of corridors, stairs and different kinds of rooms. All the pupils' books and schoolbags lie discarded in their classrooms. A teacher's coffee is still warm. When our puzzled pair try to get out again, they discover all the doors are locked. So are the windows. They can't break the glass. They can't escape. The phones are dead. Looking out the windows, they see a thick fog settling over the school...