Yeah, thinking up a story on the spot can be daunting but start small, find something that you're really interested in and branch of from there. (For eg. 4 boys searching for the lost city of Bankstown)
Once you figure out the start and end point, then you can think about what type of characters would suit your idea, and things such as characterisation and development will gradually come to you. After that you can brainstorm possible story arcs that will take place during the overarching story.
Good luck!