To manage users, log in (as an administrator, of course) and navigate to Users. You'll see a list of your existing users:
When we installed WordPress, it created only your first user (which is how you've been logging in all this time). Let's create a new user, and assign that user the next most powerful role, that of an editor. To do this, navigate to Users | Add New. You'll see the Add New User form:
As you can see, only the Username, E-mail, and Password fields are required. You can also change the Role from the default (Subscriber) to one of the other roles. In this case, I've selected Editor. Then, I click on the Add New User button.
Apart from the required fields, it's also good practice to fill in First Name and Last Name. This can make the task of further managing the user accounts much clearer.
I can repeat this process to add an author, a contributor, and a subscriber. When I'm done, the Users page (where the users can be managed) will look like the following screenshot...