To have a "following" function, you might consider using the Flag module: http://drupal.org/project/flag It allows users of your site to "flag" other users (or nodes for that matter).
Of course this doesn't subscribe them to the users they are following (even though you could list followed users content with Views module) For that functionality you might want to check out the FriendList module (http://drupal.org/project/friendlist) which integrates with a couple of other modules to probably provide the functionality you are looking for.