Generating an idea
The first step is to get all of the elements together, the genre, the technical elements... etc. Once you have these elements together, you should then start considering ideas. If you think of an idea, then move onto the "elevator pitch". Pitch you idea as if you have just walked into the elevator with someone, and you only have the time that you are both in the elevator to get your idea across, it should be simple and concise. Once you have a few ideas, write them down. While you can make the film without writing the idea down, the direction of the crew won't always be clear, and people will have different version of the idea floating around in their head. By writing it down, the whole team knows what is happening and will be able to more concisely develop a plan.
Before you start generating ideas, take a look at some of the old film entries on the 48 hours website. The winners entries can give you some inspiration about what you want to do. Some examples of idea brough up were people trapped in a building, and a group of zombies being hunted.