You should always have a cashier in your cafe in order to open it for business. You can buy a till point from the decoration menu and then choose a cashier from the avatar section.
Some cashiers are specialists and they boost your business in various ways such as shortening the celebrity arrival time or giving you more time with catering orders. You can unlock these specialist cashiers with cash and assign them to the till point you wish to use.
Check them out, they are very cool!