My recommendation is to install a browser extension that indicates when a site has a feed. Then when you are reading an article you like you can consider subscribing. Eventually you will gather a list of good sources.

Personally I use https://addons.mozilla.org/firefox/addon/feed-preview/. But many other options are available.

If you are just getting started and want to build up a list quickly:

1. Err on the side of subscribing if you are not sure. After reading a few articles it than unsubscribing is free if you decide that the feed isn’t for you (unlike email newsletters where you still somehow end up getting spam in the future). I think of adding a feed to my reader not as much as subscribing but starting a trial for that feed. If I like the content then I stay subscribed.
2. Just find new content how your normally would. Lemmy and social media are good for finding new things to follow. You can even use Google News. Once you find good content it usually has a feed.
3. You can find topic-based starter packs. Often in the form of OPML files. However I find that these often have far too much volume. But you can still start with them and then quickly unsubscribe from the feeds that don’t interest you.

Once you have a good base then you can just let it self-manage more or less. A lot of the new feeds I find are from links in my subscriptions. But if I get through my news I still go on Lemmy and often find new sources there as well.