I have been writing for forty years. Consider your plot and get a general idea of your main characters. Most importantly, figure out what type of ending you want. Will it be a possible continuation to another book or is it a one time thing? Most people write an outline. In one of my creative writing classes the teacher gave us a form to fill out where we considered each character as in individual and built on their character, their likes and dislikes, how they dressed, what type of music they likes, what they drove, where they lived, what sort of pet(s) they had. By the time I finished, I had a pretty good idea of my characters and how they would fit into the story.
Remember that the first time is a rough draft. Don't try to be perfect with it. There is more than enough time to go back and tweak it. You need to get the story line down then consider any changes you might like to make.
One piece of advice, where Science Fiction and even Fantasy are concerned: Don't get too weird. Imagination is a wonderful thing but it can carry you away. Make sure it will carry your reader as well.