My advice.
Start with the null assumption - the reader knows ABSOLUTELY NOTHING about your story going into it.
NOTHING.
NADDA.
ZIP.
The only thing you're allowed to assume is a certain threshold of intelligence. And that the reader can comprehend your level of English (or whatever language you write in 
). If you start from this point, it forces you to write your story in such a way that no one gets left behind for lacking prior knowledge.
Make it accessible to anyone.
(Emphasis on anyone. Not everyone, because that's impossible.)
Even in fan fiction I consider this important, despite the fact that your target audience likely does know some things about the world you're writing in.
On that note, you should still assume reader competence. Don't hand-hold your audience, just make sure all the pieces are there that need to be there.
One thing to remember when thinking about learning curve, or exposition in general - the reader cares more about your characters than your setting. Don't spend 20 paragraphs painting this beautiful picture of the countryside, then finally show us your farm boy milking his cow.
Oh, you've got a really cool bunch of facts about this one culture that you want to spend several paragraphs detailing to us?
WHO CARES?
No, I don't mean that in an offensive way. Please... don't give me that look. :/
Genuinely ask yourself: Who's going to care about this information? Are your readers going to care? If so, write it! If you think some of them won't care, be very hesitant. Because remember, the more you teach, the steeper your learning curve. And the steeper your learning curve, the more readers you risk losing (or gaining, in some rare cases).
Make sure you're willing to "kill your darlings," as they say. Just because you can including something doesn't mean you should. Even if your reader only glimpses 10% of your setting or world, that's okay. As long as they get the story and they want to follow along, you're golden.
Finally, info dumps.
FOR THE LOVE OF GOD, DON'T START YOUR STORY WITH AN INFO DUMP!!!
*ahem*
If you do choose to give an info dump, make sure it relates to what's happening to your characters at that moment. Otherwise, there's no real reason for the common reader to care about the information. (Just another thing to think about when you ask - who's going to care?)
This means you have to be crafty about when and where you give your information. You might realize that the reader has to know XYZ before they reach plot twist ABC in order for it to be effective. It's your job to make sure that info gets conveyed within the proper time frame.
Being a writer is in some ways like being a teacher. The less you make your teaching feel like teaching, the more your reader will learn. 
My advice.
Start with the null assumption - the reader knows ABSOLUTELY NOTHING about your story going into it.
NOTHING.
NADDA.
ZIP.
The only thing you're allowed to assume is a certain threshold of intelligence. And that the reader can comprehend your level of English (or whatever language you write in ). If you start from this point, it forces you to write your story in such a way that no one gets left behind for lacking prior knowledge.
Make it accessible to anyone.
(Emphasis on anyone. Not everyone, because that's impossible.)
Even in fan fiction I consider this important, despite the fact that your target audience likely does know some things about the world you're writing in.
On that note, you should still assume reader competence. Don't hand-hold your audience, just make sure all the pieces are there that need to be there.
One thing to remember when thinking about learning curve, or exposition in general - the reader cares more about your characters than your setting. Don't spend 20 paragraphs painting this beautiful picture of the countryside, then finally show us your farm boy milking his cow.
Oh, you've got a really cool bunch of facts about this one culture that you want to spend several paragraphs detailing to us?
WHO CARES?
No, I don't mean that in an offensive way. Please... don't give me that look. :/
Genuinely ask yourself: Who's going to care about this information? Are your readers going to care? If so, write it! If you think some of them won't care, be very hesitant. Because remember, the more you teach, the steeper your learning curve. And the steeper your learning curve, the more readers you risk losing (or gaining, in some rare cases).
Make sure you're willing to "kill your darlings," as they say. Just because you can including something doesn't mean you should. Even if your reader only glimpses 10% of your setting or world, that's okay. As long as they get the story and they want to follow along, you're golden.
Finally, info dumps.
FOR THE LOVE OF GOD, DON'T START YOUR STORY WITH AN INFO DUMP!!!
*ahem*
If you do choose to give an info dump, make sure it relates to what's happening to your characters at that moment. Otherwise, there's no real reason for the common reader to care about the information. (Just another thing to think about when you ask - who's going to care?)
This means you have to be crafty about when and where you give your information. You might realize that the reader has to know XYZ before they reach plot twist ABC in order for it to be effective. It's your job to make sure that info gets conveyed within the proper time frame.
Being a writer is in some ways like being a teacher. The less you make your teaching feel like teaching, the more your reader will learn.
