Dean Lorey, author of the Monster Academy, Johny Zombie a.k.a. My Boyfriend's Back, Happy Gilmore and Major Payne, knows a thing or two about telling a good story. He began by saying that once you see the standard structure of most of what comes out of Hollywood, you can't un-see it.
In facing a new project he suggested an easy way to know whether your plot is better for a movie or novel. Look at whether the conflict is external or internal. (Duh, but I didn't think of it until after I heard it.) Also, your hero has to have a clear goal and clean opposition. Hero's may or may not go through a story arc. Characters like James Bond and Indiana Jones don't change through their adventures and that's okay, because they've saved the world. Other character's lives are changed forever like Luke Skywalker.
Plot alone can't carry a story. Characters are very important. One great way to reveal characters is through puttng them in a position where they must choose between good and a different good. Their choice then defines them. He gave the example of Indiana Jones when he rushes in the tent where Miriam is being held by the Nazi's and finds they have a medalion that will tell them where the ark is. He pauses for a minute, weighing the choices- girl or ark. He chooses ark and puts the girl's gag back on her.
In order to fit into the time limit allowed by film, you have to tell the most story in the least amount of time. To do this, you begin at the last possible moment and end as soon as you've made your point.
For example: Neighbor knocks on the door and man opens it. She says "Ted wants a divorce." There is no "hi, how are you?" You just get right into it.
If scenes don't advance the plot, they shouldn't be there.
Common structure of a movie is-
ACT I- Length: 15-20 minutes- Life is normal when suddenly something happens that throws your character into a situation that turns their lives upside down.
ACT II- Length: 40-60 minutes- The heart of the movie has your character trying to fix whatever is broken or resolve the thing that has thrown their world apart. But as they try, they get in more and more trouble until they are hopelessly lost. In many stories the conflict shifts at mid-point when suddenly the main character realizes that the way they thought to fix the problem wasn't the right answer at all. Their new goal leads them into more danger and when all is nearly lost... then you start the next act.
ACT III- Length: 15-25 minutes- The final act has the main character reach inside themself and find a strength they never realized they possessed. Again, in Star Wars Luke puts away the guidance system and trusts the force, having character arc and story arc complete in the same moment. (He really liked that movie.)
If you have a great story idea that you want to make into a screenplay, Dean Lorry suggested that you go to http://script-o-rama.com/ and print off the script of a movie then read it as you watch it. This will give you a good idea of the basics.
Selling your script is another matter. His suggestion is to let anyone willing to read it, do so. If you think you've got a great idea, broadcast it. Also, if you can figure out the names of assistants to producers or writers, they may be more open- sort of like Junior Agents.
For me, the most helpful thing was to look at the skeleton of a story and compare it to my own plots, making sure they had clean arcs. I also have to admit that I spent a good portion of yesterday with a script in my hand watching movies. Now that was fun!