Research and Attention to Details
Research, Research, Research!
I cannot emphasize this enough, but research is the heart of believable writing. When you are writing any kind of fiction it just doesn't do your story any justice if the facts don't line up. Think about it. How many times have you watched a television show and thought "how many times has he fired that gun?" or "I don't believe a car would explode just because that happened." The difference between non-fiction and fiction is that fiction has to make sense.
For a reader to enjoy a story they must be able to temporarily suspend their disbelief in order to be entertained. Think of it like an audience watching a magician. Everyone knows that he is performing tricks and optical illusions, but what makes it entertaining is the appearance that his tricks are magical. So, the audience temporarily suspends their disbelief in magic in order to be entertained. What happens if you are watching a magician and he makes a mistake? What if you see exactly how he did the trick because he slipped up? Then it doesn't seem magical at all. If you know that the coin was in his pocket the whole time it just looks like a guy took a coin out of his pocket and pretended to pull it out from behind your ear.
As a writer you must accomplish the same magical effect, and that takes research. If you are writing a story that is set in the wild west then you don't want to make rookie mistakes like having revolvers with unlimited supplies of ammunition and horses that travel as fast as cars. It takes research to figure out that a Colt Peace Maker only holds six bullets and attention to detail to count the bullets that are expended during a written scene. Trust me there are readers out there who will catch your mistakes.
Writing Science Fiction
It is especially important when writing science fiction to pay attention to scientific principles. It wouldn't do your story any justice if you quoted Einstein's theory and wrote a well known equation down wrong. Or if you use a scientific principle for the basis of a scene and spell the scientist's name wrong. Someone will catch it.
While search engines can be very helpful for research, they cannot replace experience. Take a class, travel, get a friend to help you reenact a battle scene, etc. If you have an idea for a big "wow" moment then try it and see if it can be done. If it takes you a hundred attempts to make it happen, but it can be done, then it is a testament to your main character's skill that they can do it every time.