Here are a few good references to get started:

http://cssgridgarden.com/

https://hacks.mozilla.org/2017/10/an-introduction-to-css-gri...

https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Grid_La...

https://www.layoutit.com/grid (useful for building examples to review)

https://alligator.io/css/css-grid-layout-fr-unit/ (it's important to understand the fr unit)

Wes Bos' series is nice if you're looking for something a little more long-form/hand-holdy. I have the same problem and was working through it earlier today. Comes with exercise files for each newly introduced concept. You have to provide your email. https://cssgrid.io/

Grid: http://cssgridgarden.com

Flexbox: http://flexboxfroggy.com