New to WordPress… some general questions

Hey, I’m new to WordPress and am just going to post some random questions as they come up. For starters, I like to keep my stylsheets and scripts and images in a ‘_lib’ directory, so _lib/img or _lib/css, etc. With the themes is it mandatory to have the stylesheet named style.css and in the root directory as the theme folder? I’m just confused because I like to organize my files and directories a certain way and am not sure if I need to conform to wordpress’ organization. Also, in pages, say you have more modular content like heading, subheading, some lists, a table, etc… how do you add different edit fields so you don’t have to add all this in the main default write panel. It seems like by default there is just a heading and body content field but say i want to have a subheading field. Any advice would be appreciated.