There's a search function for a lot of it. The easiest way to discover communities currently is https://browse.feddit.de, and you'll want to copy the full URL of the community into your instance's search bar, i.e. if you wanted to follow the Pokemon TCG community you'd search for https://lemmy.ml/c/pokemontcg and it should bring up the community from there.
There's a technical reason why that is, due to the way federated servers work, but i'll defer to someone smarter than I for that explanation.