While there can be slight variations for different types of scripts (ie multicam sitcom teleplays), screenplays follow strict formatting rules.
Fortunately, software like Final Draft, Movie Magic, Celtx, and others will do the heavy lifting for you.
Here are the basics:
1.5 inch left margin
1 inch right margin (between .5 inches and 1.25 inches), ragged
1 inch top and bottom margins
Approximately 55 lines per page, regardless of paper size (top and bottom margins adjusted accordingly). This does not include the page number, or spaces after it.
Dialogue speaker names (in all caps) 3.7 inches from left side of page (2.2 from margin)
Actor parentheticals (aka wrylies) 3.1 inches from left side of page (1.6 from margin)
Dialogue 2.5 inches from left side of page (1.5 from margin)
Pages should be numbered in the top right corner, flush to the right margin, a half-inch from the top of the page. Numbers should be followed by a period. The first page is not numbered. The title page is neither numbered nor does it count as page one, so the first page to have a number is the second page of the screenplay (third sheet of paper, including the title page), which is numbered 2.
Examples of properly formatted screenplays can be found in the johnaugust.com library.