For the Presents contest you only have one chapter in which to woo the editors so you need to make that chapter the best it can be.
This means that it needs to be full of action, introduce the characters and their conflicts, dribble in the barest minimum of back story, grab the reader's attention and end with enough of a cliffhanger to make the reader want more. 'Is that all?' you ask, banging your head on the keyboard.
Don't be disheartened. There are some great resources availble on the net to help you perfect that opening chapter. Check these out to start:
eHarlequin Writing Articles
iheartpresents Competition Tips
Now don't spend all day surfing the net! You have a prize-winning chapter to write.
Just remember the golden rule of scriptwriting and you can't go wrong: enter the scene as late as possible and leave as quickly as possible.