I worked in quick serve restaurants as a manager / area manager for a good long time. The key is cleanliness, service and value.
Customers want clean places to eat with clean washrooms. This shows your kitchen is clean.
Service that is speedy and with a smile.
Value. A delicious, generous portion for the money paid. Give them a reason to spend their hard earned cash at your place, a reason why you are better than the rest. Remember their money paid to you is your reward for giving them what they want. It must be earned.
Find your competition and sit in the parking lot and traffic count, then go in and order and see what the price point is, what product you get and what improvements can be made. Don't be afraid to loiter for hours at your various competition to see what they do best and what they don't. Also look at the type of area they are in are there other restaurants, is it a commercial/industrial or residential area? Take your time and do the research.
It's not rocket science but so many restaurant start-up's fail this miserably.
We started a new restaurant and sold it for double what we paid for it 2 years later in an area where most other restaurants failed. I went on to help many restaurant owners to be just as successful. Well the ones that listened anyway.
Good Luck!