The definition of science fiction from Wikipedia: "Science fiction is a genre of speculative fiction typically dealing with imaginative concepts such as futuristic science and technology." So it's basically some (really amazing) made-up science/tech stuff.
You can get inspired and think up some ideas by observing your environment and visual artwork you find on the internet. Make up the characters in your story and create brief backgrounds for them, give them their personalities, give them a physical description, etc.
Something I like doing for the story lines is to just write (rough draft) and put any ideas for events that come into my mind onto the paper (but make sure everything fits in with the previous ideas). When I'm done, I make a plot chart for it and reread, and mark out anything I don't like.
When revising, make sure to look at grammar rules and for variety in your sentences. Word your sentences clearly.
p.s. I'm only okay at writing. But if you're really into writing stories, really plan them out and put thought/hard work into them, because you will be proud of them in the end.