Register your screenplay with the Writer's Guild of America and establish a proper copyright. It protects your rights should the studio decide to produce your script.
Contact the studio or agency through which you intend to submit the manuscript and ask them for formatting guidelines. Most have specific rules for how a screenplay should look--font, spacing, format and even paper stock. Adhere to their guidelines rigidly and make sure you shift formats (if necessary) when sending your manuscript to more than one studio.
Follow standard printing procedure. Unless the studio or agency stipulates otherwise, you need to print your screenplay on white bond paper, 8.5 inches wide by 11 inches tall. Print the screenplay in black ink on one side of the paper only, and maintain a margin of 1 inch around all sides of the paper. Most studios require you to use 12 point Courier font. There's more to these specifics than just uniformity. Each page of a screenplay should constitute one minute of screen time, and the specifications are intended to maintain that ratio.
Double-check your screenplay for any copy editing errors--the spellchecker is rarely enough--and use standard copy formula for Hollywood screenplays. For example, settings are usually written in all-caps, while the term O.S. stands for "off screen."
Print the manuscript out on a laser printer similar high-end machine.
Bind your screenplay pages using brass brads and use a piece of white card stock as the first and last page. The card stock should be the same dimensions as the paper itself; print the title of the movie on the front piece of stock, along with your name, contact information (or the contact information of your agent if you have one), and copyright notice.
Mail the screenplay to the proper department (as stipulated by the studio) or hand it in yourself if appropriate.