You can walk through the "help wizard", or click "list" link in topnav to see all of the known instances.

EDIT: I failed to mention, that you should not feel like joining one instance excludes you from the greater federation of networks...you can join one instance and follow people from anywhere else. And, if you begin to dislike the instance that you joined, simply move to another one...or even better - though a tad more work for you - set up your own personal instance, do whatever you want, and still be able to follow anyone on any other instance.