Account creation
Creating an account is easy. First you choose an instance and then you register with your email and password.
How to choose an instance
Check out Instances for a list of instances that are waiting for you to join.
If you don't find an instance for you there, there are more sites like that on the bottom of that page.
If you don't know what software you want to use yet you might want to, go back to What are Fediverse projects?.
Once you found an instance, before you create an account, make sure you get familiar with the instance rules and are okay with them.
Too few or soft rules could mean you're getting a lot of unwanted messages or posts on your local or global feeds.
Also be aware if the instance says it's a free speech instance. For most people that is not the good thing it sounds to be.
Creating your account
All you need to create an account is a username and a password. Depending on the instance and the software it is using, you might need an email address (some software like Mastodon force this requirement, but for others like Pleroma this is optional or configurable by the admin). Usually, no phone number or other form of identification is needed.
You can choose any username that doesn't exist on that instance yet.
So even if someone else on the Fediverse is already called @anonymous you can still call yourself @anonymous if you're on a different instance.
Your Fediverse handle will be @username@instance.domain and everybody on the Fediverse will be able to find you with that handle.
Further reading