To setup and assign User roles go to Settings > Users. Each user in the list have switch right to their name that determines will they have admin role or not. When switched on, user will be set as administrator in your org. If not, user will have limited - non-admin access.
Admin users can access all workflows and see all Deals, Contacts and all Settings options. Non-admin users are restricted to Workflows they are assigned to, to seeing only deals where they are being marked as Deal owners or assigned as team member to it. Will not have access to organisation settings and there are many custom options that organisation owner and administrators can choose on what they can see and do.