Finding the idea:
This is what I do. I look into my imagination, take everything that interests me as a person... all those fantasies I have/had, and those are my basics. For my script "Story Of A Hitman" I took my fantasy of wanting to be a professional killer/gun for hire, and wrote it around a character based on me. I took that, developed a dramatic "happening" (any will do really) and then based everything that happened to the character on that dramatic turn. Just like in real life, anything that happens to a character changes their direction in life. It affects their state of mind, their aspirations and their view. For better or worse.
There are some basic storylines that you can work off. Those being: Hero and Villain, Boy falls for girl, etc... it all starts with a basic line. This is where you should start, if your fantasies aren't similar to mine. Just write something that you WANT TO MAKE. What kind of film have you always thought you'd like to make? Or, what is your favorite film? As with most things, storytelling isn't all that original anymore. Just about every aspect of any basic story has been told. The best you could do as a storyteller, is take the best bits of any idea you have... and mash them together.
Developing the Idea:
So you have your bits and pieces written down. Or your one line that will basically sum up your script. Now comes the fun part... turning that line into a developed script!
No doubt you've got your hands on a word processor, or some form of script writing program... but incase you don't, grab Celtx and install it. Start writing notes in a blank text document, such as characters (you should ALWAYS HAVE A MAIN CHARACTER!) and locations... extra bits that might make your story nice and meaty. Got your main character? Good.... now comes the next bit.
Start a new script project, and think of the first place we should meet your character. We should introduce him/her to the audience, either during a dramatic moment... or through dialog (voice overs are a great way to start). In most cases, it'll be smooth sailing from here on. Just write what comes to mind, and it'll all piece itself together!