USEFUL INFO
Screenplay format: the rules that matter
Format looks like a formality and is in fact a unit of measurement: a properly laid-out page is worth about a minute of film.
Why it exists
Standard format comes from production: it exists to count. How many pages, how many minutes, how many shooting days, how many night scenes.
It is also why a personally formatted text worries readers even when it is well written: it means the numbers can't be run.
The parts of a page
The scene heading, in capitals: interior or exterior, where, when.
The action, in the present tense, only what can be seen and heard. The character's name, in CAPS on their first appearance. The dialogue, in a narrower column in the middle. An expression written in brackets.
The rules that matter
A twelve-point monospaced typeface, Courier New or similar.
No camera moves, unless you are the one directing the film. Nothing going on in someone's head that can't be seen.
Which software to use
Dedicated software handles the layout for you: Final Draft is the industry standard, and that is the one I would recommend.
Whatever you use, deliver a PDF: it is the only way the page arrives as you left it.
One last thing
Writing a screenplay is, in a sense, subjective. You do have to respect some rules, but here you also have to put something of your own into it.
There are no set rules for that part. If the intention is expressed intelligently and, above all, if it is clear, then it works. Don't tie yourself in knots.