Ok, you want to be a director. You've never been on a set. You've never directed. But you have a script, a camera, and some actors. You have twelve dollars for a pizza, and you want to make your first short film.
Great! Let me tell you one truth: when you're on set, especially a location you talked someone into giving you for free, you're going to be trying to rush. And you're going to set yourself up for failure in post. If you can follow a few simple guidelines, you'll already be 10 steps ahead of the next new indie short film.
Have a shot list
This means go through your script and figure out what you're going to shoot. You can add or drop things on set, but have a plan to start with. Divide your script into scenes, and number them.
Slate your shots
If you're recording sound separately (like with a DSLR setup, sound into a recorder), you need to do a marker for syncing. Make sure you do this on camera AND for audio. The obvious way is to use a clapboard, making sure all cameras are actually rolling and looking at the clapper. Also, make sure to verbally state the scene, shot and take: Scene 25A, Take 1. Oh, and if you move the camera, say from a wide to a closeup, the shot changes: Scene 25B, Take 1.
If at all possible, have a script supervisor
Or someone, anyone, looking at the script. This person can make sure the slate is correct and that you're getting all your intended shots. When you're a new director on a tiny crew with no budget, you're doing the job of everyone and you tend to forget things. Try to get someone else on hand to help with this.
Get more than one take of each setup. There's a good chance that your actors will find their stride after the first take. Sometimes they nail it, sometimes they don't.
Don't get too much coverage
Ten takes of the same setup is probably overkill unless someone just absolutely keeps missing a mark or something. "Tape is cheap" but everyone's time isn't, and they're probably doing this for free. Too much coverage makes the edit much more difficult.
Don't roll in between takes
There's nothing worse than hunting through a ten minute clip for the 2 takes you ran. If you cut, then cut. Continuous takes are okay sometimes, particularly when resetting takes a second and you just want to try to quickly grab the shot while the actors are in the heat of the moment or something. But a majority of the time, you should be cutting.
Just try your hardest to not let things get way from you. If you or anyone on your set says "fix it on post", backhand them. The sooner you learn, the better! This is your set! Enjoy the control while you have it, because we all know the film is really made by the editor. Ha.