The easiest way to go about this is to visit a website like www.dailyscript.com or www.simplyscripts.com and download the scripts for a couple of mainstream movies. Then, read them from cover to cover.
Now visit your local video store and rent the DVD of the scripts you just read. With script in hand, put the disc in to play and STUDY at the very least, the first ten or fifteen minutes of the film. Actively compare it to the script. Pay particular attention to the opening image that sets the story in motion or signals ground zero for the hero's story line.
The next thing you should really examine is the way the script moves the action from scene to scene. NOTICE on the page the 'slug lines' (INT: APARTMENT - DAY) and the often brief descriptive text that follows. Pay attention to the DIALOGUE both as it's written; but also, how it's delivered by the actors. And finally pay attention to the PACING - how quickly the film & script move the action along as they 'set up' the story and the main character's quest.
The more screenplays you read, the more comfortable you will find yourself with the strict formatting required by the film industry, once you sit down to put your own great idea onto the page.
Good luck with it - and let me know when you've completed your script. I'd love to read your work.