The advice given to a beginning screenwriter is write something original. Write several scripts of your own and see where your story leads beforecoping one on a scriptwriting system. The best advice given is to just do it, have faith in yourself.
You should have your work read aloud by other actors so you can pin point what needs to be done to make the script better. Some more good advice is to get your story published before you turn it into a movie because it gets you more recognition and then more than likely your book will sell more when you turn it into a movie, whereas it wouldn't have if you just jump straight into making it a movie.
The best way to get the attention of a director or star is to make sure that your script has memorable characters, the story must play out visually, and it needs a "hook" that will make it promotable.