Either as a waiter at a restaurant or a bartender, both are OK.
I wanna have dinner in a restaurant.
We had dinner at a restaurant.
I once worked in a restaurant.
The women work in a restaurant.
In the U.S., you have the option, when you enter a restaurant, to sit in the smoking or non-smoking section.
My father runs a restaurant.
Last Friday I ate dinner with my girlfriend in a restaurant.
It was last Friday that I ate dinner with my girlfriend in a restaurant.
It was with my girlfriend that I ate dinner in a restaurant last Friday.