Maybe you already have a very clear image of what you want to write. If so, power to you. If you have not such a clear image, power and pictures to you to get the image you want. I’m going to try and put down some help here for you. Help that I think might be good advice, although opinions, as mileage, might vary.
This is a tricky one sometimes, because you may want to write a vampire-zombie romance in a science fiction setting. Nothing wrong with that, but before you have a go at it, see what you feel is the most important part of your story. Vampires? Zombies? The romance? Or is it more the science fiction side of your story?
What I mean here is: don’t muddle your story with too many genres. They can end up a confusing mess for your readers, and that’s probably the last thing you want (although I take no responsibility on that). Confused readers won’t promote your work. Crossing boundaries in genres is a great thing, but there is such a thing as too much.
Tense and view
Decide on a tense (past tense, present tense) and a view (first person, third person, third person overview, omnipresent) and stick with that. It’s too easy to flip from one to the other but you’ll have a ball editing all that back into one persistent story. Make sure you understand the possibilities and limitations of the form you choose. Therefore make sure you know how you want your story to look and base your form (tense, view) on that. If you write a story that has many action lines going in many places, picking a first person view isn’t the smartest thing to do (although it’s possible).
Write complex. Or not.
Who are you writing for? How do you want people to read your work?
If you write for children, don’t write too many long sentences, don’t overuse long and complicated words. If you write for a general adult public, consider the level of literacy and the range of their vocabulary. If someone needs to look up three words per chapter to make sense of your story, there’s a good chance they’ll toss the book aside. On the other hand, if someone breezes through your work without encountering any intellectual challenge, guess what will happen?
Keep you work consistent in that too. Don’t go from simple to complex to simple. That’s annoying.
Tie up your open ends. Don’t expect readers not to notice something that’s not ‘resolved’ in your story. Perhaps half of them won’t, but the other half will. Remember: contrary to real life, life inside a book has to make sense. No open ends. (Unless you intend to resolve those in a sequel.)