You may want to find a good writing software program. There are ones that are free and ones that cost, make sure to do your research on the programs or you can choose, software programs are not necessary to write and can you choose to write without one. Take your idea or story and write it down, from beginning to the end, this will allow you to know how you want your story to flow. You will need to make sure you know the format for writing a play as you will need to understand the formats for TV's, Features and New Media as they may be a little different regarding acts and scenes.