Re: Background
You could try concept mapping. Write down aspects of your character on paper in "clumps" then draw lines between the clumps that have something to do with each other. Write over the lines why they have something to do with each other.
Oh, and for practice you might want to start off with an established, published character that you know quite a bit about. Maybe writing up a background for such a character that has oodles and oodles of little details that you know about from being entertained by him/her/it will help you understand how to frame your own character's background.
That Why thing is definitely important but the trick is to know what the "Whys" are. One of the things that will help out in writing background is if you nail down what your character's supporting cast looks like. Enemies, Allies, Friends, Family, Rivals, Co-Workers and the like are part of your character's background. If you write down a small paragraph description of each and what their function(s) are in telling the character's story then you have done a great deal to establish a background with great depth.