Welcome to Mastodon, new #twittermigration peeps! The first answer to a question you will have is "No, it doesn't matter which instance you join...at first".

Get started, learn how the interface works, and then look for instances that match your interests. Think of it like Twitter Communities. You can migrate your followers and redirect users to your new instance after you sign up for your new account. So don't worry too much about your first choice.

Feel free to ask any questions!