The first source is a website called W3Schools. W3Schools is extremely helpful because it has every little tutorial about CSS, be it font sizes or colors or how to pinpoint your HTML to your CSS.
The second source is a video on YouTube by Derek Banas. He goes thoroughly indepth with CSS and it's always nice to see someone do it before your eyes to get a hang of something.
The third source is a tutoring/tutorial site called Codecademy. Codecademy is a website where you can learn how to code in real time, including CSS. I've used this several times to help advance my CSS coding, because it also has tutorials you can follow on your own time.