Invent the rules of your universe. As a sci-fi writer, you'll need to think about things that writers of realistic fiction don't--like what kinds of physical laws will govern your imagined world. For instance, is time travel possible? Can characters read each others' thoughts? Can vehicles travel at the speed of light? Part of the fun (and challenge) of writing a sci-fi book is nailing down these details.
Think of a scenario that engages the fictional universe you have imagined. Chances are that a story will begin to reveal itself to you as you hash out the workings of the world your characters are going to inhabit. Many sci-fi books involve some type of quest element--a hero needing to rescue another individual, or the universe itself, from destruction--but that doesn't mean your story has to.
Create narrative drive. While it's fine to spend time dwelling on the fascinating physical aspects of your fictional world--that's part of the joy of the sci-fi genre--don't forget that readers need a reason to turn the page. A well-turned love story involving compelling characters is one of the best ways to achieve this, but almost any engrossing situation will do.
Do your research. If your subject is a cutting-edge field like nanotechnology, or if your work involves speculation and theorizing, make sure you are starting from solid scientific ground before you expand on your theories. Serious sci-fi readers are likely to discount your work otherwise. Successful sci-fi scenarios are firmly rooted in a grain of truth, then extrapolate out from there.
Take advantage of the online sci-fi writer community for support and tips as you work. eBookCrossroads.com has a database of links helpful for sci-fi writers, including workshops and sites useful for research (see Resources below).
Check out how-to books by veteran sci-fi writers that take you through the nuts and bolts of the writing process. Be sure to adapt any advice to your individual style, however. Two to try are "How to Write Science Fiction & Fantasy" by Orson Scott Card and "How to Write Tales of Horror, Fantasy and Science Fiction" by J. N. Williamson, both available at Amazon (see Resources below).
Read as much sci-fi as you can. This is an integral part of your research, plain and simple. You have to know what makes good sci-fi before you can write your own.
Proofread, edit and rewrite extensively. Get notes on your story from a trusted friend or another writer or editor, and incorporate them.
Submit your work to as many publishers as possible. Expect lots of rejection letters, but don't give up.