Action! Lights! Camera! ... Damn It!

I have started writing a screen-play that I've had in my head for a long time now, and am coming across some problems. I've read other screen plays, devoured them even (okay, not literally), tried to learn the format, and I'm having difficulty.

I'm good with the way dialogue is supposed to look and feel, it's just everything else that's giving me trouble.

Does anyone know a helpful piece of scriptwriting software that is free? Anything you have personal experience on, or even at this point just heard something good about would be great.

Thank you!