As a whole the French are fashion forward and style is their way of life. Paris itself is an exceptionally stylish city and all flip-flops, athletic wear out, short shorts and American-type casual outfit are not taken pretty seriously at all in the capital of fashion.
Some elegant simple classic clothes (pair of trousers, a black dress, jeans, simple sweaters and button downs and a nice pair of shoes depending on the season) are the most suitable clothes there.
A light jacket is necessary for cooler summer evenings (the weather is rather unpredictable in Paris).
In winter warmer clothing, woolen sweaters and pullovers, thick socks and a good winter coat are needed.
The outfit must always be spiced up with accessories like belts and scarves.
As to shoes, a good pair of shoes (for example, flats for women) to walk in and explore the city are quite a good choice in warm weather. A pair of boots is a must-wear to keep feet warm and dry in winter. Dress shoes are required only for going out in the evening.