When you start making films, there are a bunch of things that film makers need to avoid. Here are some of the TOP ones:
Don't "act" a part. This is film, and unlike stage acting, we are not accustomed to "suspension of disbelief". You need to BE who you are portraying. If you need an adult, you MUST have an adult play the role. If you are male, don't put on a wig to play a female. If you are 15, don't pretend to be 10. If you are not a private investigator, don't try to be one. You get the idea...
Don't use "pretend" locations. If you wrote a doctor's office into your script, then you need to film in a doctor's office (or something that looks like a doctor's office). Don't have access to one? Rewrite your script. If it isn't real, or you can't make it look real, don't film it.
Don't use fake props. If it doesn't look 100% real, don't include it in your film. Don't use guns.
Don't smile when you're not supposed to smile. This kills a film fast. Find something to focus on that will get you serious. Re-shoot until you get it right.
Don't look at the camera.
Don't talk to yourself. Don't. Really - don't do it. It will kill your film. Let the audience figure it out. The less you tell, and the more you "show" the story, the better it will be.
Don't ever use text to tell us what's happening. That worked during the silent film era in the 1920's. This isn't the 1920's. Don't do it. SHOW the story with the camera and sound. Along the same line, DON'T show texting and DON'T use voiceover. These are quick and easy ways to ruin a film. You can show someone who is texting, but don't show the screen for any more than a single line text. If you need to show the screen, show it briefly where we can't easily read the texts.
Don't write a script about something you haven't experienced. Write what you know. If you are a student, write about what it's like to be a student. Comedians make a lot of money by telling stories about everyday events from an interesting (and funny) perspective. It's all about the perspective.
Don't put extra scenes in your film. Include only what you need to tell the story without confusing your audience. Keep your story-lines simple. Use a single location whenever possible.
Don't include extra dialogue. Remove any extraneous dialogue from your script.
Don't break the basic film rules. No jump cuts; don't cross the line; follow the rule of thirds; film cutaways,
Don't break the basic audio rules. Test your equipment first! Keep the mic consistently 1M from the actor;
Don't break the basic editing rules: Cut to action; use cutaways; show reaction shots;
Don't use your real names and don't wear a school uniform (as discussed in class).
Don't use zoom shots. Use camera movement to get what you want.