I think one of the hardest things to do in the beginning is get a reader settled in a world. Even writing contemporary, you still have to lay down a setting and world that makes sense and feels real, not like we're watching people on stage. The faster you can make a reader feel comfortable with your world, the easier it will be for them to get sucked into a story.
No matter what type of book you are writing, there's probably a setting. Maybe it doesn't play a huge role, or maybe the setting is almost a character itself. Either way, you need to make sure the setting is understood. Readers don't like to float in the nether too long—grounding a reader in your novel is essential.
So how do you do that? Well, there are lots of ways. There are also lots of way to confuse a reader, too. As with most things in writing, there needs to be a balance (which is why we spend so much time editing).
Focus on setting details that are "important." As a writer, we tend to know most everything about our novels, right down to what things look like. But that can also mean we might go over board in description. Now, don't get me wrong, having a lot of description isn't necessarily bad, but you have to think about what matters most. When you choose to describe something, you're saying that it has significance in some way. Be mindful of that. Describe what matters most to your characters, what stands out in the scene, what might foreshadow future events in the book.
Let's try a random example. Say you're describing a street lamp. This description can be either important or superfluous depending on your story. If it's just your ordinary street lamp, maybe it doesn't merit extra description, but what if it's your character's first time in a new city? What if she notices how bright the lights are in comparison to the country? Or how strange the craftsmanship is compared to where she is from? Then it might be something to signify to the reader that this place is foreign, a setting detail worth talking about.
Another major thing to always consider is what I call "immediate relevancy." (I will likely be repeating this in the other sections because it is that important.) When building a world, the information you give must always be needed at that moment, otherwise it has great potential to confuse or bore a reader. Like say you're talking about these weird street lamps when the MC has not seen one yet—instead of being interesting, it'll be, "Uh, what's with the street lamp conversation? Weird." You don't need to "prep" your reader for these uber weird street lamps, just mention them when they get there. In fact, you'd be surprised how littler prep a reader needs—trust that they will follow you just fine.
Writers of fantasy/sci-fi/paranormal, etc. have a special task in building a world that is different from our own, and that world comes with rules the reader must understand, and hopefully understand quickly. If there is magic, the guidelines must be explained. If there are flying ships, there must be a feasible reason. If there's time travel, it has to be believable in that world. It can be a challenge to have a reader buy your world, to believe in it.
Again, here it's important to give information as it becomes immediately relevant. Listing off all the rules to the magic system in the first chapter just isn't going to cut it. First, it'll be boring, and second those rules will be forgotten because they'll have no significance to the reader yet. Not to say you can't have rules in the first chapter, but just like introducing characters, it should be a gradual thing.
If possible, showing the rules and world should be the go to. Don't just say "These are the penalties for using this type of magic." Show your MC losing her eyesight for a day because she cast a certain curse. That makes the world memorable and the rules clear.
But in this showing, you have to be careful to avoid the "As you know, Bob" pitfall. This is when characters explain things they should already know, and it's pretty obvious that it's being explained for the reader's sake only. It can be tricky to avoid these moments (and you'll notice a lot of characters are novices at something because it makes things easier to explain since you learn along with them), but doing so makes for a stronger beginning and a more authentic world/character set.
Lots of people are afraid of backstory (Thanks to the criticism of the flashback, I think.), but no story is complete without at least a little. Stories don't happen in a vacuum, and no matter what your world is like, something happened before your story and something will happen after. Sometimes the things that happened before are a surprisingly vital part of the characters' current struggles, and you need back story to get across the full impact of your novel.
Again, I will repeat the importance of immediate relevancy. With backstory it is most important, otherwise readers will ask, "Why are you telling me this?" It has to be clear why, and usually then the reader won't even notice that you've stopped the forward motion to tell them about the past. I've seen very long passages of backstory that work just fine in novels because that information was essential to understanding either the plot or the characters' motivations.
But like most anything in a story, backstory can go awry as well. It's one of those things that we as writers can get carried away with if we're not careful. I personally try to use it sparingly; usually if you only use it when absolutely necessary, you won't go overboard. It shouldn't be an excuse to add extra detail—it should be used as a tool to move the story forward.
Ultimately, world building in the beginning should make a reader "comfortable" with your world. That's not to say that the story has to start off calm, only that world details should always be clear and concise and helpful to a reader. Confusion can be not only frustrating, but it can cause distrust in you as an author. If something doesn't make sense, or if an obvious question isn't answered, a reader loses faith. I wish it weren't true, but it is. Having a solid world, whether it's on a magical plane or in Clovis, CA, is an essential part to a solid beginning.
Now, that ends the series, but if you have any questions feel free to ask in comments. I'll be checking often.