Research your story. Unless you are already knowledgeable about all things science, spend a good deal of time researching the science behind your story. If you're writing about traveling from galaxy to galaxy at the speed of light, you need to understand how fast the speed of light is and the time it would take to travel from one galaxy to another. You'll also want to know the names of the galaxies, the actual distance between them and what forms of life might be possible in those galaxies. You can create entirely fictional galaxies, but even fictional galaxies need to be accurate in the context of real science as we know it.
Outline your story. You don't need to write a step-by-step outline of every single event that will take place in your story, but write at least a rough outline. Be sure to tie the science of your story into the outline. Characters and story plot in your book revolve around the science, not the other way around. Science is the star of your literary show. Write an outline that helps you keep the science in focus by weaving it seamlessly into the plot.
Be creative with any alien characters you plan to use. Avoid making alien creatures that have too many human characteristics. If this is your intention, you can write human-like aliens, but for the most part, these aliens will seem trite and not realistic. Think outside the box when you create aliens. They don't need to eat with a mouth. Maybe they absorb food through their skin. Maybe they don't eat at all. The same goes for the worlds you create around your aliens. Think away from how things operate here on Earth. A planet in another galaxy will very likely have nothing in common with the way life works on Earth. Tie the environment of the planet in with the alien characteristics you create. Aliens living on a planet that is void of light will likely not see in the traditional sense.
Write a first draft of your story before you read it through. Capture the flow of your story as you work from the outline. When you've finished a first draft, go back and read your story for clarity, accuracy of facts, and mechanical errors. Do a second draft to make additions or deletions to your story and to correct any factual errors or anything you feel won't get by your readers, then consult the "Writer's Digest Market List" for publishers looking for a good science fiction novel.