To practice working with selectors, complete the CSS Diner game.
Next, a great way to learn CSS is to try to rebuild a few common websites. Although these exercises may seem tedious, there is NO better way than to rebuild existing sites. You might be tempted to just look at the source code, and that is fine for inspiration, but really try to do this yourself!
Sites to rebuild:
http://code.tutsplus.com/tutorials/the-30-css-selectors-you-must-memorize--net-16048