Okay, that's actually a lot of questions... Let's start with the paper verses computer angle: Try writing a few paragraphs in pen and a few on your computer. Whichever feels more natural - that's how you write.
As for content, don't worry about the number of chapters. As you progress through the book, you'll get a feel for when a break is coming. Follow your gut. Publishers generally look for about 65 to 70 thousand words for a first effort. But, don't get discouraged if you don't hit the magic number. Your first draft will probably be far below that, then you go back and fill in.
ABove all, I give this bit of advice: Write a story for yourself before anyone else. Lose yourself in the adventure of writing, and you will end up with a great book!