If it's an ethnic restaurant in an urban area, a simple black restaurant chair is the beginning of a minimalist style to pursue for your restaurant's ambiance. These chairs are lightweight, quiet to move, and easy to replace, should they need replacing. They work with whatever interior design you decide to pursue and they are relatively inexpensive.
You could also consider family style seating, which usually consists of long tables and individual benches. Of course, this depends more on the size of your restaurant and the type of atmosphere you'd like to create. These restaurants usually tend to be louder, since they are less intimate.
If it's a diner, you will need the standard restaurant chair as well as the booth, the staple of late night coffee talks and bustling mid-town luncheons. Comfy, soft, and made for patrons to sit and order again and again, it's a standard for any restaurant that aspires to recreate nostalgia, kitsch, or homey feeling.
When you move away from the pure restaurant, you immediately think of bar chairs. Quality bar chairs are ones that spin, and thus allow conversation to open up and out. That brings people closer to the bar and makes for a more fun environment. Commercial bar furniture usually includes a set of tables and chairs, which can be easily bought from a restaurant furniture supply, but if your restaurant also has a lounge space, which necessitates quieter, softer furniture, you need to find accommodating furniture; something between bar chairs and soft booths.
If you are going for a lounge, you will need low furniture with lots of cushions. Big square blocks which could function as ottomans or seats are also good. And if need be, you can even make do with a set of restaurant chairs and tables, as long as you have plenty of candles to place on top of them.
Of course, these are just tips to help you open your restaurant. Once things are off and running, you can refine the style to include certain unique elements that make your restaurant stand out from the crowd. Your commercial furniture is one important aspect of how to do it.