just to add my advice as well - know your characters. Not just "she is 5 11, slim, hot, big boobs, dark hair " *drooling* .. but her personality, how did she develop that personality, who are her parents, how was she at school, was she a drama queen, or a book worm. Pretty much create a brand new person.
Come up with a story. Simple story. Don't go christopher nolan s**t on us. See how this character will react to the situations that you'll place them. How do they interact with one another..
writing a screenplay is not as easy as I thought would be first time I started learning. It takes LOTS of practice.
As far as creating the story: look around you. Write everything that is in your head on A4 type of paper. There is a reason why you want to make a movie, or write a screenplay. As you're writing crap down you can read it back, and maybe you can come up with something.
Let me try. Yogurt napkin knife fettuccini laptop chair plant curtain..
Ok, how about this short film: A cougar from the local zoo broke free, and harassing the neighborhood. One man, who never leaves his house, and only lives through internet (pretty much a nerd) doesn't know about incident. Cougar breaks into the man's house. He has to face the challenge to either break his old habits or become a neighborhood hero. FIN.
Simple and stupid, but you got the idea..
Now, i'm back to my yogurt and fetuccini.