Establish a goal for your protagonist. This should be fairly simple, especially if you are adhering to the conventions of your genre. If you're writing a horror film, the protagonist's goal is probably to get home safely; if you are writing a romantic comedy, the protagonist's goal is probably to find love. If you understand what the protagonist is looking for, you can create conflict by giving him obstacles to clear in order to find it.
Create external conflict by giving the protagonist an obstacle. In a crime thriller, you could have the detective pick up a witness who refuses to talk; in a horror movie, you could set the story in a maze or similarly fiendish area. External conflict exists when characters have to overcome obstacles outside of themselves.
Create internal conflict by creating events that alter the characters' perception of the world. Imagine a drama, for example, where the main character is graduating from university and is set on medical school. However, an external conflict causes him to rethink this decision (maybe he walked by a community center and realized he wanted to be a social worker). The character's decision-making process is an example of an internal conflict -- rather than having to overcome obstacles to achieve a goal, he has to redefine what his goal was in the first place.
Mix external and internal conflict to write a screenplay that addresses both how people deal with events and how people deal with their perceptions of themselves. This will help make your film more engaging and easier to relate to.