It's simple. What you do is that before you go to sleep (preferably when you're lying down in your bed), think about whatever thing you think you might have a nightmare about/spooked you out today. This will cause it to not be in your dream. It usually can't be a broad subject. For example, if you read a bunch of creepy stories, you need to at least briefly remember each one for a second or so.
The reason this works is because when you sleep, your brain sorts out whatever things happened during that day, and some of those things end up in your dreams. By thinking about them right before you go to sleep, your brain doesn't go over them as you're sleeping, thus causing them not to be in your dream. (note: There might be other reasons, but this is the main one.)
Try it, it works.