Word of Advice: Know What You're Talking About

The title sounds simple enough, but you should all pay a lot of attention to what it's saying. Fiction is, by default, about making things up. The characters you create aren't real, though they may be based on real people, and neither is the world, even if it is just like our world. One problem I see a lot, however, is that people write about locations and places, about peoples and cultures, and come off as if they don't actually know anything about those things.

So a huge rule of thumb is this: If you don't know anything about it, research it. If you still don't know anything about it after researching, research more. If after researching and researching over and over you still don't know anything, assume you never will and don't write about that.
You can't write a story using complex quantum physics if you don't know a darn thing about quantum physics. People who know something about it will know, and those that don't know a lot will likely question. The same can be said if you want to write about a Brazilian character living in Brazil. If you don't know anything about Brazil, or about it's culture, why are you writing about it? You have to be conscious of how such a culture thinks, feels, believes, etc. If you look at a lot of Canadian literature, for example, you can see the differences between the Canada and the U.S. or even the U.K., despite our cultures being remarkably similar, in general. The differences are small, but it does serve to establish that we don't think exactly the same.

When writing, you have to be certain that you are being believable, even in fantasy (unless you're being weird for comedic effect, which is okay too). Your characters have to reflect who or what they are. If your character is American there are certain things you should be aware of. For most of us here at YWO that shouldn't be a problem, so I'll take it from another angle. If you're writing about a character who lives and is native to the U.K. you have to be completely and fully aware of what it means to live there. You can't just make it up off the top of your head, because you'll likely be horribly wrong. You have to know, even if you want to write something that might be alternate history, because even then you will find that similarities are likely to exist. The same goes if you're writing about China, Russia, or anywhere.

So the first step to making your fiction believable and creating believable characters and addressing them from their cultural perspective appropriately.