Just the other day I was watching a movie with my wife. I thought that the movie had potential, but I kept getting really confused during a good portion of it. Terminology, technology, concepts, and relationships that I didn't understand or had never heard of kept popping up. I was getting lost in trying to figure out some of the minutiae instead of enjoying the storyline of the film. This is a problem that I have begun to see in movies as well as in books. The writers don't seem to want to give any background early in the story. So when should you and how much should you give? There are some times when giving the back-story is part of the story itself. You don't always want to tell everything up front. Instead, you want to have it divided out because it helps to drive the story. That can work out wonderfully. The youth novel Holes wouldn't have been nearly as good if the entire background of the main characters was given in the beginning. It became a driving force in the storyline. If that is how you plan to have your story flow, go right ahead. However, make certain that you are dealing with characters, situations, and terminology that is familiar to the average person. You do not need your reader to get bogged down in trying to figure out what on earth you are talking about because you are using unique terminology. I will admit to having closed books and turned off movies because I wasn't able to figure out what they were talking about. If telling the background isn't a driving force in your story, then I would suggest getting that background out there as soon as possible. Your reader wants to know whose lives they are following or what situation they are watching unfold. Even Star Wars gave enough background to understand that there was a civil war taking place and that the Rebels were desperately trying to find some advantage to use against the much larger and stronger Empire. If you can't learn from Star Wars, who can you learn from?
The next thing that I find useful to point out as part of the background is enough information to educate your reader. A very dangerous trap that I have seen many writers fall into is that they assume that the person reading their book has a similar education to themselves. I'm not talking about college degrees here. I am referring to practical experience that impacts the terminology that you use or the processes that you might choose to not explain because you are so accustomed to them yourself. The safest bet is to never assume that your reader is familiar with what you are talking about. Just because a reader has chosen a military adventure doesn't mean that they know the difference between a carbine and a lmg. Just because someone chooses to read a legal thriller doesn't mean that they know what an indictment is (a large number of people don't). Some writers are concerned that they might make their audience think that they are stupid if they explain everything. If that is your concern, then have an ignorant character. Very often the people involved in different situations have no experience with whatever is going on. Let the character ask the questions that the audience might have. The audience doesn't get lost and you have a new, useful character for your story.
Be imaginative. Create your own stories. Create your own people. Create your own universe. Just remember that your readers, your audience cannot see into your imagination. You have to use your talent for story telling to draw them into your imagination. It's what some of the greatest stories do.