Did you know that there's a catalogue on the coffee table next to the couch where you can buy equip-able items that eg. give you more time, increase customer patience? this should help.
if you feel like you're supposed to be doing something but don't know what, i would walk around and click on customers to see if you missed their order. I think you should go for the basic ending first, so don't even worry about talking to the chef or taking out the trash until you've got the regular gameplay down.
also, if you've got a lot of customers, i remember collecting all of their orders first, then turning in the tickets at the knob thing, and when an order is ready, hit the knob again before grabbing it. stay at the ticket turn-in thing until there's a reason to leave (meal ready, customer ordering < you can wait for a meal to be ready before you go to get customers orders)
(btw it will take one of each type, so if you have a meal and side ticket, it will take both, but if you have 3 appetizer tickets, you will have to go back for each one. you can see which tickets you have on the top right, and if you forget, you can see how many meals each person has had based on how many plates they have on the table. if they don't have a menu, they're waiting for food.)
i hope some of this helps