Most websites, blogs and forums have an RSS feed. You can follow them in Friendica just by adding their RSS feed

• for most websites you can add /feed or /rss to the end of the URL
• for forums (and if the above fails) you can usually find the rss feed by looking at the source for the webpage (right-click and choose view page source
• if you don't feel like nerding out you can use a Chrome extension to get the feed such as Get RSS Feed URL

• Once you had the feed URL you can just paste it in the search bar (where the big magnifying glass is) and hit the Follow button

you won't see anything immediately because the feed will only show new posts, not posts that were posted before you followed