Here are some places where you can find out more about CSS Syntax, Selectors, Properties and Values. Nobody can remember everything about CSS Syntax, how to apply it etc. Your goal should be to know some of the main terms and ideas so that you can more quickly find the information you need in a reference.
The Mozilla Developers Network (MDN) and CSS Tricks are two great web sites to go to for help. I have found it can be helpful to start with a trusted resource like these instead of googling first and having to sift through a bunch of stuff.
Mozilla Developers Network: MDN
This site sets the standard as a reference for all things web.
CSS: Cascading Style Sheets
This is the main area for all things CSS at MDN
This is an excellent resource not only on CSS but also for front-end web development in general.
CSS Tricks Homepage
CSS Ruleset Terminology
A ruleset is what applies CSS to an element or elements. This is the core syntax for CSS.