More seriously, if you fancy some reading, I wrote a free HTML and CSS tutorial: http://marksheet.io/
Before you dive into specific frontend feature like Flexbox or Grid or even a full CSS framework, I believe you should first understand the basics of markup and styling, which are very simple but not easy.
Because otherwise, you will start using tools you don't fully understand, and will struggle extending or fixing your design.
After that, you can simply Google whatever style you want to implement, or problem you're facing, "just in time", and end up on CSS Tricks or Stack Overflow.