There are several pages that keep curated instance lists and sort them by category. One of the criteria for inclusion is that the instance must not be too large: e.g. FediGarden and JoinFediverseWiki.

However, you should perhaps consider in advance whether Mastodon is really the right access to the Fediverse for you or rather one of the other services such as Friendica, Calckey, Pleroma, etc. Especially if you are used to a clear structure like on Lemmy, Mastodon can be quite confusing because it is microblogging without context. The instances are connected to each other, no matter what software they are running. The Jointhefediverse.net can help you get started.