Before jumping into the action of a scene, there are two important things I've noticed screenwriters don't necessarily do. They don't describe the setting the first time we see the scene and they don't describe what the character looks like. Without giving a brief description of the scene and characters, it's hard to visualize the script in our minds.
Take a look around you. Where are you? Find three key details and write them down. When you have a new scene, it's important to describe the setting and what it looks like so we get an idea of the environment of the world the story takes place in. A setting can give an idea of the character's financial situation. However, if it's just a small scene, then there's no need to describe the setting. Here's an example of how to set up a scene.
INT. LIVING ROOM - DAY
A flat-screen television set hangs on the wall across from a worn couch. A speck of sunlight briefly lights the room.
The following example straight away shows three key details right away and doesn't go into too much detail. If you have a simple setting, such as a living room, make sure to make it unique and imaginative. Take a look at what you wrote down and see if you can simplify it a bit more. If you have a big setting and there are lots of key details, you can start by describing the scene, cut to the action, and have the character walk through the scene and describe what we'd see along the way during the action.
Now character descriptions are different. When you first introduce a new character, it's important to have their name in caps, followed by their age, and have a brief description of what they look like, then show the action they are doing. Without giving the reader an idea of what the character looks like, we won't have an idea of who they are or why we should care about the person. Unless it's important to the story, you don't have to describe their race. Take a look at the following example on how to describe a character.
ANDREA, 32, long blonde hair, dressed in a tank top and torn jeans, scrubs hand sanitizer on her hands.
Take a piece of paper and describe someone you know. Who are they? What do they look like? What action would they be doing? If this sounds difficult, practice describing yourself by looking into the mirror. Find a few key details about yourself and write it down.
If you have any questions, feel free to send me an e-mail and I'll be more than happy to help you out!