It is possible for you to define resources 'on the fly' (when you are assigning resources to tasks). But it is usually easier to first define human, equipment, and facilities resources while you're in the Resource Spreadsheet (Window, Resource), in case you need to customize the new resource's calendar. When you first create the resource, the calendar automatically created for it will look exactly like the Master Calendar. Get into the habit of customizing each new resource's calendar (Options, Calendars, select the resource), as soon as you've created the resource.