You can check out Daniel Shiffman’s chapter on phyaics engines in Nature of Code which he has made available for free.
Personally I didn’t quite understand that tutorial, so if you are like me you might want to read my little tutorial at eeyorelife.github.io and if you have any questions or feedback don’t hesitate to ask.
The link to my website also provides a link to examples that is very valuable when trying to learn to code with Box2D. Good luck!