Coming up with a basic plot or idea for a story may take some thought. Brainstorm your ideas by writing all of them down, even if they sound ridiculous. If you don't hinder your thoughts or block thoughts you think sound stupid, you may end up with an idea that you wouldn't have otherwise. Write down your ideas and then weed through them, choosing many you think are "doable." Then, expand on each one to help weed the selection down even further by realizing which you like better than others.
Don't choose to write a script about astrophysics if you don't know anything about astrophysics, and that goes for many other topics as well. When you write what you know, there is lesser chance of making a mistake or an inaccuracy. A script that is based on what you've experienced in your own life or what you've studied personally will have more subconscious emotional ties. You'll write with more descriptions and more feeling, since you know exactly how to describe certain things.
While a brainstorming session will help you develop many new ideas, you may also come up with other ideas along the writing process and often when you're not at home writing. Carry a tape recorder with you at all times, so that you can record new plots and thoughts about the script whenever they come into your head.
After you've finished your first draft of your script, re-read it and make edits. You may even decide to rewrite the entire thing and that's OK. After you've done that and have your second draft, re-read it and rewrite it again. Good scripts often go through 10 or 20 rewrites before they are even shown to outsiders because no matter what you've written, it could probably be better.
No matter what you think you know about writing, there's always more to learn. Considering joining a local writers' group where you can get honest feedback on your writing as well as see how others approach the craft. There are many workshops and classes (including local community colleges and adult exention classes) on screenplay writing, as well as dozens of books on the subject. The more you know about the process, the better your finished script will be.