Don’t worry! Just sign up on this server, it’s good well-run general server that is especially suitable for new people:
…and if you want a tiny bit more choice, here are a couple more equally good general servers suitable for new people:
If you want even more choice see the full Fedi.Garden server list.
During the sign-up it asked me why I want to join. What do I say?
Some servers may ask you why you want to join, but don’t worry! Just very briefly say you 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.