Just start where you think would be best place. You know the characters best, so put them in a position that is fitting. Remember to build up suspense, otherwise you may run out. Have some action, some drama, anything you want to incorporate. Just remember, it is your story so have fun with it. ;)
Start with an exposition- introduce the characters and setting.
Always start in the middle of the action for a fanfic. People have a lot to except for a fanfic and if your first chapter is boring they'll assume the rest will be.
Creating the mood and setting. Maybe some starting story dramatics.
Attention grabber!!!!!!! Always always always have a brilliant attention grabber! :D