The formatting of your screenplay will determine the fate of your screenplay, No one will take you seriously with a script that does not look professional.
On your title page, you should have a title, who wrote it, and which version of the draft it is currently. Additionally, if it is based on something else, you should put that under the authorship.
Note: Only state it is based on a book, comic, etc., if it is published.
The meat of the screenplay is the slugline, character, dialogue, and action.
The slugline is essentially a short description of the scene. INT (Interior) and/or EXT (Exterior), the location (i.e. THE MALL), and time (i.e. DAY or NIGHT) are mandatory for a complete slugline.
The character is the person in the scene. The character’s name is capitalized over the dialogue and it is also capitalized in the action line when the character first appears.
Dialogue is simply what character speaks in the scene. It is located under the character’s name.
The action line describes the action in the scene (i.e. The wind was blowing, John runs, etc.).
Here are some screenwriting resources that have helped me:
Formatting your screenplay can get complicated, but luckily you can use software to minimize to effort.
Final Draft is the industry standard software for screenwriting and, as I write this, Final Draft 10 is $249.
Celtx is a popular alternative to Final Draft. It is a free, browser-based program that a lot of people (including me), swear by.
Note: There are different subscription tiers you can get with Celtx that include shot lists, scheduling, and more, but the free version is great also.
I hope this was helpful!