To integrate offsite data within Discourse you are likely to need a plugin to do it properly so you can cache data server side and schedule updates so you don’t have to delegate this to the client which will slow page loads considerably.
I would start by understanding how to write plugins: Beginner's Guide to Creating Discourse Plugins Part 1: Creating a basic plugin
Knowledge of the theming system is very useful: Developer’s guide to Discourse Themes
The widget system: A tour of how the Widget (Virtual DOM) code in Discourse works
The Wordpress API if your source website is Wordpress: REST API Handbook | WordPress Developer Resources
Nokogiri for scraping: https://nokogiri.org/