Take a look at some good resources:

• fedi.tips an infomrmal, unofficial guide for non-technical people who want to use Mastodon and the wider Fediverse.
• fedi.directory. If you were on the Internet in 1998, you know how Yahoo! organised all the known web sites into categories. Yeah, this is that for the fediverse.

• twitodon Twitodon will scan your Twitter “following” list, checking to see if any of those accounts have also used this service.