I think the best approach is playing the long game with the 'All' feed because lemmy is so small, which is the approach that I've taken.
Basically you'd be blocking all the communities that don't interest you, foreign language servers you can't understand, and a couple prolific bot accounts reposting everything from reddit. That approach will take a good long while to weed out everything you don't want to see and be left with things you like and weird and varied discussions that are fun to engage in. This works in your favor in the long run because new communities crop up on your feed now and then that you'd never come across otherwise.
That being said, I recommend any of the asklemmy or ask (group) communities for good interaction and reading material, and the million cat communities that make your feed whether you want it or not
I like subscribing to every community that piques my interest. even if they don't appear active. I still find new and interesting communities by browsing "All"
you can use the filter communities search bar, and type in keywords that interest you. Or use the drop bar and browse the newest communities that have been created. or search by top subscribers in the whole fediverse for highly active communities.
I also liked browsing different instances - looking for an instance to create an account, but also learn instances that cater to themes, like anime, countries, nsfw, books, etc.
If you mean you can see the community but there are no (or very little) posts, look in the side-bar for the local subscribers stat. If it's 0, that's why. If someone subscribes, new content will federate.
It would be nice if there was a tool to find these communities that'd be useful for smaller instances, but I wouldn't know if there is one.