How can i use selectors to layouts

Hi!
I have some concerns about a homework in my class. This Homework is about apply some selectors to the layouts and don´t do long the stylesheet.

I'm learning now about html and css. How can i use the selectors in layouts or divs, for example this selestors
http://net.tutsplus.com/tutorials/html-css-techniques/the-30-css-selectors-you-must-memorize/,
to apply this selectors to layouts without text,they just have colors.
(and i learning english, excuseme for the orthography)