The screenplay is so important that words cannot do this issue justice. If the screenplay is weak all your other efforts will be in vain; excellent filmmaking technique is necessary but not sufficient. People watch movies because they want a journey, human warmth, and catharsis, not because they like your editing or lighting. If your screenplay has a believable, compelling character who has a difficult problem to solve, you're in good shape. Structure is the key to a successful screenplay. Almost a century's worth of practical filmmaking experience has shown that effective screenplays have a three-act structure: act 1 is the beginning, or the set-up; act 2 is the middle, or confrontation; and act 3 is the end, or resolution.

Click here to view Writer/Director Jason Reitman discussing scripts.

Many screenplays are organized into a 3-act structure. The tradition of writing in this form comes from the theater and was followed by filmmakers. Think of it as a foundation for building a house that others can easily identify, even if the details are new and original.

In any case, don't forget that the three acts don't necessarily have to occur in chronological order – as Jean-Luc Godard said, “every movie needs to have a beginning, a middle and an end, but not necessarily in that order.”

Film makers need to really explore their idea before they start to write. Work out what you would like to say and how you are going to say it. A film maker also has think about who is this film going to appeal to.

In terms of writing the script there's a number of things to consider :
The story is vital and is at the top of this list for a reason. Without a good story all your filmmaking efforts will be fruitless. Beautiful lighting, creative camerawork and smooth editing are pointless if the story isn't compelling. Why else would anyone want to watch the movie?