begin with installing firefox, the web develper add-on and the firebug.
and then get used to work with them.
most css is formatted in style.css of the theme.
and make it a habbit to look up online tutorials with anything you are not sure abeout - for instance http://www.w3schools.com/
and refresh the browser after changes to css, as the browser cache often holds on to 'old' stuff: press CTRL and F5 at the same time, or use the reload button.
at last, but certainly not least, there is this unread documentation, called the codex - http://codex.wordpress.org/Main_Page -
intimidating, maybe, but i suggest that you do the same as with css and html, if in doubt, search and read.
and the default theme as a source of 'how did they do it' - as is any other (free) theme.
if you like a feature of a theme, just download it, take it apart, and learn from it.