Don’t worry! Just sign up on the server linked below, it’s a good well-run general server that is especially suitable for new people:
If you want to choose your server from a list of many good servers organised into categories, languages, locations etc, have a look at the Fedi.Garden directory:
All of the servers listed on Fedi.Garden have promised to uphold specific standards of technical reliability and responsible moderation.
During the sign-up it asked me why I want to join. What do I say?
Some servers may ask why you want to join, but don’t worry! Just very briefly say why you want to join. e.g. to post cat pictures or whatever. It’s not an in-depth question or competition, the server is asking because it helps prevent spam bots.