Plain size 12 Courier or Courier New font.
No other fonts, no italics, no bold letters or digits. Nothing like that. Nowhere. Never.
The First Page
No page number on page one.
You may put the title of your script at the top of page one, but not on any of the following pages. If you decide to put it on page one, it must be centered, capped and underscored.
You should begin with FADE IN: flush at the left margin. Almost all screenplays do that. Some readers take this very serious.
It follows a master scene heading or narrative description.
You may alternatively start your screenplay with BLACK SCREEN followed by words to be superimposed on the black screen. That will look this way: