One of the best ways to pick up any new framework is to go to the official documentation/site and open up the Getting Started guides. For express, they start here: Installing Express. You can keep progressing through the various sections, and stop when you think you can set up a basic backend that can serve your HTML & CSS files, as well as connect to a database.
You can also use examples at expressjs repository as a reference if you want to solve a particular problem with Express.
A lot of web development will look like this — you have to learn a new framework — so you'll go to the official docs and try to get an idea of what it is, how it works, and how it differs from existing things you know. One of the more beautiful parts of this field is that new libraries and frameworks are popping up all the time, and you'll have to re-learn a lot of things all the time to keep up. This is why I think learning how to learn effectively is more important than learning a particular framework. Good luck, and if you have any questions, ask below!