Choose your target audience. Before you can start writing, you need to know who is reading your work. In other words, you must decide in which genre your novel or screenplay is placed. It's important to know your reader or viewer when crafting your prose or else you risk losing their interest.
Make a rough outline of your book or screenplay. One of the biggest problems new writers face is not having a planned out plot to follow when writing. To avoid getting sidetracked, it's best to develop an outline of your plot before you start writing. Write the main points of the plot down with brief, one or two sentence summaries of each main point.
Develop your main characters. If you don't know your characters inside and out, you can't expect your audience to know them, either. The easiest way to create nice, round characters is to make a character chart for each of them. A character chart is simply a profile that explains the important facts about each character, such as age, appearance, interests, family and history.
Write your first draft. Don't worry about perfection on your first draft. The important thing here is to develop your story according to the outline you created earlier. It's okay to change things from your outline if you come up with a better idea or find that a certain plot point doesn't work, just make sure that the story makes sense.
Write a second draft. This is the time to go through and edit your story. Your second draft should end up being shorter than your first. If a scene doesn't add to the story, delete it.
Write a third draft, and a fourth draft, and so on until your story is perfected. Sometimes it only takes a couple drafts before a book or screenplay is complete. Sometimes it's not until the 10th draft that the author feels his work is ready. It's up to you to decide when you have a finished product.