How do i start ?

I like the stopping a bomb scenario best. I would recommend that in order to try for top marks, you spend some time researching Bomb Disposal teams - how they go about their work etc. You could even quote what research you did at the end of the story (ie "with thanks to xxx for their useful insight into bomb disposal workers".) That will show you've put extra effort into your story - after all, it's how 'real' authors work.