Read plenty of science fiction. During this process you will develop a strong sense of the genre conventions. You'll find the things that are overused, and you'll learn the kinds of stories that are most popular at the moment.
Create a future world that's internally consistent and believable. You can set your story in the near future, or you can set it thousands of years beyond our current time. Either way, you need to imagine every conceivable element of your future society, from politics to technology.
In his book "Writing Popular Fiction," novelist Dean Koontz says that developing a strong background is more important in sci-fi than in any other genre.
Write everything down. Sit at your computer, or curl up with a notebook and start taking notes. Write a detailed history of your world.
Figure out what kind of story works best in your world. Science fiction tales can deal with anything from adventure to mystery and romance. Dean Koontz lists eight basic plot types, but within each there are literally thousands of possible variations, and some stories are actually a combination of different types.
The kind of story you tell can develop naturally from the realities of your world, or it can exist as a counterpoint to create contrast.
Create characters to populate your story. Write detailed character histories.
Find a way to pass information along to your reader. It can be difficult to explain your future reality without slowing the story to a crawl. There are many proven methods for doing this, but the general rule is to keep the action moving and don't make yourself too obvious.
In his book "On Writing," novelist Stephen King urges a hard-driving approach to cranking out stories. Start Writing and keep writing every day until you finish your first draft. This is very important, because if you let a day go by without writing something down, you will lose momentum.
The amount you write is not as important as making constant progress, so even if you can only write one page per day, keep going.
Once finished, put your story away and don't look at it for a few weeks. Stephen King says it's very important to let your mind refresh so you can go back to your story later and see it with new eyes and more perspective.
Read your first draft and then let some close friends read it. Listen to their suggestions to find out if your story is working. During your own read, make notes about any weaknesses in your story.
While you're writing, you may have a particular reader in mind. Stephen King mentions his wife, Tabitha, as his ideal reader, and she is one of the first to offer criticism of his initial draft.
Take the feedback from the previous step and apply it in a rewrite.
Let your story rest again.
Read through one more time and polish your writing as you go. This will be your final draft. Your story should now be ready for submission to publishers.