To get a sense of what a script page looks like, look at this page from The Avengers, shared here for educational purposes.
Notice the amount of white space in relation to the text. We aren't looking for over-written pages, but keeping everything tight and simple.
A simple ratio to consider is that one page should equal one minute of screen time. This means that the descriptions, dialogue, and actions on the page should take one minute if viewed.
Here is the Full Avengers script (PDF) for viewing
Bonus Material: The Origins and Formatting of Modern Screenplays, from Filmmaker IQ
Elements of a Screenplay
Four major elements comprise the construction of a screenplay:
And lastly, there is formatting for THE END.
Most scriptwriting programs will help you with this formatting.