An idea is a start, but it's not a story until you have whipped it into a plot (the sequence of events in a story).
A standard recipe for a plot is as follows: start with a problem, stir in complications and conflict, top with a climax and finish off with a resolution. A plot has three main ingredients: beginning, middle and end.
The beginning
The beginning of a story needs to be strong enough to whet the reader's appetite for what's to come. By the end of the first page, the reader should know who the protagonist is, what the story is about, and with what conflict the main character will struggle. A good way to grab the reader's attention is to start in the middle of an action or with a conversation that hints at conflict. Take a look at this beginning:
'I'm getting out of here!' shouted Jack as he slammed the door behind him.
Tim looked up from his book. 'Where are you going?' he asked.